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Augusta Ada King, Countess of Lovelace.

Preface

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Welcome to the Ada Programming tutorial at Wikibooks. This is the first Ada tutorial covering the Ada 2005 standard. If you are a beginner you will learn the latest standard — if you are a seasoned Ada user you can see what's new.

Current Development Stage for Ada Programming is "(Jun 2, 2008)". At this date, there are more than 200 pages in this book, which makes Ada Programming one of the largest programming wikibooks^[1].

But still there is always room for improvement — do help us to expand Ada Programming. Even beginners will find areas to participate.

About Ada

Wikipedia has related information at Ada (programming language)

Ada is a programming language. Suitable for all development needs, it has built-in features that directly support structured, object-oriented, generic, distributed and concurrent programming.

Ada is a good choice for Rapid Application Development, Extreme Programming (XP), and Free Software development.

Ada is named after Augusta Ada King, Countess of Lovelace.

Programming in the large

Ada puts unique emphasis on, and provides strong support for, good software engineering practices that scale well to very large software systems (millions of lines of code, and very large development teams). The following language features are particularly relevant in this respect:

• An extremely strong, static and safe type system, which allows the programmer to construct powerful abstractions that reflect the real world, and allows the compiler to detect many logic errors before they become bugs.
• Modularity, whereby the compiler directly manages the construction of very large software systems from sources.
• Information hiding; the language separates interfaces from implementation, and provides fine-grained control over visibility.
• Readability, which helps programmers review and verify code. Ada favours the reader of the program over the writer, because a program is written once but read many times. For example, the syntax bans all ambiguous constructs, so there are no surprises, in accordance with the Tao of Programming's Law of Least Astonishment. (Some Ada programmers are reluctant to talk about source code which is often cryptic; they prefer program text which is close to English prose.)
• Portability: the language definition allows compilers to differ only in a few controlled ways, and otherwise defines the semantics of programs very precisely; as a result, Ada source text is very portable across compilers and across target hardware platforms. Most often, the program can be recompiled without any changes.^[2]
• Standardisation: standards have been a goal and a prominent feature ever since the design of the language in the late 1970s. The first standard was published in 1980, just 3 years after design commenced. Ada compilers all support the exact same language; there are no dialects.

Consequences of these qualities are superior reliability, reusability and maintainability. For example, compared to programs written in C, programs written in Ada 83 contain ten times fewer bugs, and cost half as much to develop in the first place^[3]. Ada shines even more in software maintenance, which often accounts for 80% of the total cost of development. With support for object-oriented programming, Ada 95 brings even more cost benefits, although no serious study comparable to Zeigler's has been published.

Programming in the small

In addition to its support for good software engineering practices, which are applicable to general-purpose programming, Ada has powerful specialised features supporting low-level programming for real-time, safety-critical and embedded systems. Such features include, among others, machine code insertions, address arithmetic, low-level access to memory, control over bitwise representation of data, bit manipulations, and a well-defined, statically provable concurrent computing model called the Ravenscar Profile.

Other features include restrictions (it is possible to restrict which language features are accepted in a program) and features that help review and certify the object code generated by the compiler.

Several vendors provide Ada compilers accompanied by minimal run-time kernels suitable for use in certified, life-critical applications. It is also possible to write Ada programs which require no run-time kernel at all.

It should come as no surprise that Ada is heavily used in the aerospace, defence, medical, rail road, and nuclear industries.

The Language Reference Manual

Wikipedia has related information at ISO 8652

The Ada Reference Manual (RM) is the official language definition. If you have a problem, if no one else can help, and you can find it, maybe you should read the RM (albeit often a bit cryptic for non-language-lawyers). For this reason, all complete (not draft) pages in Ada Programming contain links into the appropriate pages in the RM.

This tutorial covers Ada Reference Manual — ISO/IEC 8652:1995(E) with Technical Corrigendum 1:2001 and Amendment 1:2007 — Language and Standard Libraries, colloquially known as Ada 2005 or just Ada.

You can browse the complete Reference Manual at http://www.adaic.com/standards/05rm/html/RM-TTL.html

There are two companion documents:

• The Annotated Reference Manual, an extended version of the RM aimed at compiler writers or other persons who want to know the fine details of the language.
• The Rationale for Ada 2005, an explanation of the features of the language.

The Ada Information Clearinghouse also offers the older Ada 83 and Ada 95 standards and companion documents.

The RM is a collective work under the control of Ada users. If you think you've found a problem in the RM, please report it to the Ada Conformity Assessment Authority. On this site, you can also see the list of "Ada Issues" raised by other people.

Ada Conformity Assessment Test Suite

Wikipedia has related information at ISO 18009

Unlike other programming languages, Ada compilers are officially tested, and only those which pass this test are accepted, for military and commercial work. This means that all Ada compilers behave (almost) the same, so you do not have to learn any dialects. But because the Ada standard allows the compiler writers to include some additions, you could learn a cool new feature only to find out that your favourite compiler does not support it...

Programming in Ada

Getting Started

Where to get a compiler, how to compile the source, all answered here:

• Basic Ada — Read Me First!
• Finding and Installing Ada
• Building an Ada program

Language Features

These chapters look at the broader picture, introducing you to the main Ada features in a tutorial style.

• Control Structures
• Type System
• Strings
• Subprograms
• Packages
• Input Output
• Exceptions
• Generics
• Tasking
• Object Orientation
• Memory Management
• New in Ada 2005
• Containers
• Interfacing
• Coding Standards
• Ada Programming Tips
• Common Programming Errors

Computer Programming

The following articles are Ada adaptations from articles of the Computer programming book. The texts of these articles are language neutral but the examples are all Ada.

• Algorithms
  • Chapter 1
  • Chapter 6
  • Knuth-Morris-Pratt pattern matcher
  • Binary search
• Error handling
• Function overloading
• Mathematical calculations
• Statements
  • Control
• Expressions
• Variables

Language Reference

Within the following chapters we look at foundations of Ada. These chapters may be used for reference of a particular keyword, delimiter, operator and so forth.

• Lexical elements
  • Keywords
  • Delimiters
• Operators
• Attributes
• Pragmas

Predefined Language Libraries

The library which comes with Ada in general and GNAT in particular. Ada's built-in library is extensive and well structured. These chapters serve as a reference for Ada's built-in libraries.

• Standard
• Ada
• Interfaces
• System
• GNAT

Other Language Libraries

Other libraries which are not part of the standard but freely available.

• Libraries
  • Multi Purpose
  • Container Libraries
  • GUI Libraries
  • Distributed Objects
  • Database
  • Web Programming
  • Input/Output
• Platform
  • Programming Ada 95 in Linux
  • Programming Ada 95 in Windows
  • Programming Ada 95 in Virtual Machines (Java, .NET)

External resources

• Open-source portals
• Web Tutorials
• Web 2.0

Collections

Printable Versions

The following are collection pages. All collection pages are comprised of groups of the already available pages. You can use them for printing or to gain a quick overview. Please note that those pages are partly very long.

Tutorial 

Show HTML (1,839 kb) — Download PDF (2,663 kb, 243 pages)

Keywords 

Show HTML (470 kb) — Download PDF (290 kb, 59 pages)

Operators 

Show HTML (232 kb) — Download PDF (189 kb, 27 pages)

Source Code

The Source from the Book is available for download and online browsing. The latter allows "drill down", meaning that you can follow the links right down to the package bodies in the Ada runtime library.

References

Further reading

Ada 2005 textbooks

• John Barnes (2006). Programming in Ada 2005. Addison Wesley. ISBN 0-32-134078-7.
• Mordechai Ben-Ari (2009). Ada for Software Engineers (Second Edition with Ada 2005). Springer. ISBN 978-1-84882-313-6.
• Alan Burns, Andy Wellings (2007). Concurrent and Real-Time Programming in Ada. Cambridge University Press. ISBN 9780521866972.
• Nell Dale, John W. McCormick (2007). Ada Plus Data Structures: An Object Oriented Approach, 2nd Edition, Jones and Bartlett. ISBN 0763737941.

Manuals and guides

• Ada Quality & Style Guide: Guidelines for Professional Programmers (wikibook)
• Rationale for Ada 2005, by John Barnes (2007)
• Ada 2005 Reference Manual (2007)

High-Integrity Software

• ISO/IEC TR 15942:2000, Guide for the use of the Ada programming language in high integrity systems. ISO Freely Available Standards [3]
• ISO/IEC TR 24718:2005, Guide for the use of the Ada Ravenscar Profile in high integrity systems. ISO Freely Available Standards [4]
• John Barnes (April 2003). High Integrity Software: The SPARK Approach to Safety and Security. Addison-Wesley. ISBN 0-321-13616-0.

External links

Authors and contributors

This Wikibook has been written by:

If you wish to contribute as well you should read Contributing and join us at the Contributors lounge.

Basic Ada

"Hello, world!" programs

"Hello, world!"

A common example of a language's syntax is the Hello world program. Here is a straight-forward Ada Implementation:

with Ada.Text_IO;

procedure Hello is
begin
   Ada.Text_IO.Put_Line("Hello, world!");
end Hello;

The with statement adds the package Ada.Text_IO to the program. This package comes with every Ada compiler and contains all functionality needed for textual Input/Output. The with statement makes the declarations of Ada.Text_IO available to procedure Hello. This includes the types declared in Ada.Text_IO, the subprograms of Ada.Text_IO and everything else that is declared in Ada.Text_IO for public use. In Ada, packages can be used as toolboxes. Text_IO provides a collection of tools for textual input and output in one easy-to-access module. Here is a partial glimpse at package Ada.Text_IO:

package Ada.Text_IO is

   type File_Type is limited private;

   --  more stuff

   procedure Open(File : in out File_Type;
                  Mode : File_Mode;
                  Name : String;
                  Form : String := "");

   --  more stuff

   procedure Put_Line (Item : String);

   --  more stuff

end Ada.Text_IO;

Next in the program we declare a main procedure. An Ada main procedure does not need to be called "main". Any simple name is fine so here it is Hello. Compilers might allow procedures or functions to be used as main subprograms. ^[1]

The call on Ada.Text_IO.Put_Line writes the text "Hello World" to the current output file.

A with clause makes the content of a package visible by selection: we need to prefix the procedure name Put_Line from the Text_IO package with its full package name Ada.Text_IO. If you need procedures from a package more often some form of shortcut is needed. There are two options open:

"Hello, world!" with renames

By renaming a package it is possible to give a shorter alias to any package name.^[2] This reduces the typing involved while still keeping some of the readability.

with Ada.Text_IO;

procedure Hello is
   package IO renames Ada.Text_IO;
begin
   IO.Put_Line("Hello, world!");
   IO.New_Line;
   IO.Put_Line("I am an Ada program with package rename.");
end Hello;

"Hello, world!" with use

The use clause makes all the content of a package directly visible. It allows even less typing but removes some of the readability. One suggested "rule of thumb": use for the most used package and renames for all other packages. You might have another rule (for example, always use Ada.Text_IO, never use anything else).

with Ada.Text_IO;use Ada.Text_IO;

procedure Hello is
  
begin
   Put("Hello, world!");
   New_Line;
   Put("I am an Ada program with package use.");
end Hello;

use can be used for packages and in the form of use type for types. use type makes only the operators of the given type directly visible but not any other operations on the type.

FAQ: Why is "Hello, world!" so big?

Ada beginners frequently ask how it can be that such a simple program as "Hello, world!" results in such a large executable. The reason has nothing to do with Ada but can usually be found in the compiler and linker options used — or better, not used.

Standard behavior for Ada compilers — or good compilers in general — is not to create the best code possible but to be optimized for ease of use. This is done to not frighten away potential new users by providing a system which does not work "out of the box".

The GNAT project files, which you can download alongside the example programs, use better tuned compiler, binder and linker options. If you use those your "Hello, world!" will be a lot smaller:

 32K ./Linux-i686-Debug/hello_world_1
8,0K ./Linux-i686-Release/hello_world_1
 36K ./Linux-x86_64-Debug/hello_world_1
 12K ./Linux-x86_64-Release/hello_world_1
1,1M ./Windows_NT-i686-Debug/hello_world_1.exe
 16K ./Windows_NT-i686-Release/hello_world_1.exe
 32K ./VMS-AXP-Debug/hello_world_1.exe
 12K ./VMS-AXP-Release/hello_world_1.exe

For comparison the sizes for a plain gnat make compile:

497K hello_world_1 (Linux i686)
500K hello_world_1 (Linux x86_64)
1,5M hello_world_1.exe (Windows_NT i686)
589K hello_world_1.exe (VMS AXP)

Worth mentioning is that hello_world (Ada,C,C++) compiled with GNAT/MSVC 7.1/GCC(C) all produces executables with approximately the same size given comparable optimisation and linker methods.

Things to look out for

It will help to be prepared to spot a number of significant features of Ada that are important for learning its syntax and semantics.

Comb Format

There is a comb format in all the control structures and module structures. See the following examples for the comb format. You don't have to understand what the examples do yet - just look for the similarities in layout.

if Boolean expression then
   statements
elsif Boolean expression then
   statements
else
   statements
end if;

while Boolean expression loop
   statements
end loop;

for variable in range loop
   statements
end loop;

declare
   declarations
begin
   statements
exception
   handlers
end;

procedure P (parameters : in out type) is
   declarations
begin
   statements
exception
   handlers
end P;

function F (parameters : in type) return type is
   declarations
begin
   statements
exception
   handlers
end F;

package P is
   declarations
private
   declarations
end P;

generic
   declarations
package P is
   declarations
private
   declarations
end P;

generic
   declarations
procedure P (parameters : in out type);

Note that semicolons consistently terminate statements and declarations; the empty line (or a semicolon alone) is not a valid statement: the null statement is

null;

Type and subtype

There is an important distinction between type and subtype: a type is given by a set of values and their operations. A subtype is given by a type, and a constraint that limits the set of values. Values are always of a type. Objects (constants and variables) are of a subtype. This generalizes, clarifies and systematizes a relationship, e.g. between Integer and 1..100, that is handled ad hoc in the semantics of Pascal.

Constrained types and unconstrained types

There is an important distinction between constrained types and unconstrained types. An unconstrained type has one or more free parameters that affect its size or shape. A constrained type fixes the values of these parameters and so determines its size and shape. Loosely speaking, objects must be of a constrained type, but formal parameters may be of an unconstrained type (they adopt the constraint of any corresponding actual parameter). This solves the problem of array parameters in Pascal (among other things).

Dynamic types

Where values in Pascal or C must be static (e.g. the subscript bounds of an array) they may be dynamic in Ada. However, static expressions are required in certain cases where dynamic evaluation would not permit a reasonable implementation (e.g. in setting the number of digits of precision of a floating point type).

Separation of concerns

Ada consistently supports a separation of interface and mechanism. You can see this in the format of a package, which separates its declaration from its body; and in the concept of a private type, whose representation in terms of Ada data structures is inaccessible outside the scope containing its definition.

Where to ask for help

Most Ada experts lurk on the Usenet newsgroups comp.lang.ada (English) and fr.comp.lang.ada (French); they are accessible either with a newsreader or through one of the many web interfaces. This is the place for all questions related to Ada.

People on these newsgroups are willing to help but will not do students' homework for them; they will not post complete answers to assignments. Instead, they will provide guidance for students to find their own answers.

For more online resources, see the External links section in this wikibook's introduction.

Notes

1. ↑ Main subprograms may even have parameters; it is implementation-defined what kinds of subprograms can be used as main subprograms. The reference manual explains the details in 10.2 LRM 10.2(29) (Annotated): “…, an implementation is required to support all main subprograms that are public parameterless library procedures.” Library means not nested in another subprogram, for example, and other things that needn't concern us now.
2. ↑ renames can also be used for procedures, functions, variables, array elements. It can not be used for types - a type rename can be accomplished with subtype.

Installing

Ada compilers are available from several vendors, on a variety of host and target platforms. The Ada Resource Association maintains a list of available compilers.

Below is an alphabetical list of available compilers with additional comments.

AdaMagic from SofCheck

SofCheck produces an Ada 95 front-end that can be plugged into a code generating back-end to produce a full compiler. This front-end is offered for licensing to compiler vendors.

Based on this front-end, SofCheck offers:

• AdaMagic, an Ada-to-C translator
• AppletMagic, an Ada-to-Java bytecode compiler

Commercial; proprietary.

AdaMULTI from Green Hills Software

Green Hills Software sells development environments for multiple languages and multiple targets, primarily to embedded software developers.

GHS claims to make great efforts to ensure that their compilers produce the most efficient code and often cites the EEMBC benchmark results as evidence, since many of the results published by chip manufacturers use GHS compilers to show their silicon in the best light, although these benchmarks are not Ada specific.

GHS has no publicly announced plans to support the new Ada standard published in 2007 but they do continue to actively market and develop their existing Ada products.

DEC Ada from HP

DEC Ada is an Ada 83 compiler for OpenVMS. While “DEC Ada” is probably the name most users know, the compiler is now called “HP Ada”. It had previously been known also by names of "VAX Ada" and "Compaq Ada".

• Ada for OpenVMS Alpha Installation Guide (PDF)
• Ada for OpenVMS VAX Installation Guide (PDF)

GNAT, the GNU Ada Compiler from AdaCore and the Free Software Foundation

GNAT is the free GNU Ada compiler, which is part of the GNU Compiler Collection. It is the only Ada compiler that supports all of the optional annexes of the language standard. The original authors formed the company AdaCore to offer professional support, consulting, training and custom development services. It is thus possible to obtain GNAT from many different sources, detailed below.

GNAT is always licensed under the terms of the GNU General Public License.

However, the run-time library uses either the GPL, or the GNAT Modified GPL, depending on where you obtain it from.

Several optional add-ons are available from various places:

• ASIS, the Ada Semantic Interface Specification, is a library that allows Ada programs to examine and manipulate other Ada programs.
• FLORIST is a library that provides a POSIX programming interface to the operating system.
• GDB, the GNU Debugger, with Ada extensions.
• GLADE implements Annex E, the Distributed Systems Annex. With it, one can write distributed programs in Ada, where partitions of the program running on different computers communicate over the network with one another and with shared objects.
• GPS, the GNAT Programming Studio, is a full-featured integrated development environment, written in Ada. It allows you to code in Ada, C and C++.

Many Free Software libraries are also available.

GNAT GPL Edition

This is a source and binary release from AdaCore, intended for use by Free Software developers only. If you want to distribute your binary programs linked with the GPL run-time library, then you must do so under terms compatible with the GNU General Public License.

As of GNAT GPL Edition 2008:

GNAT Modified GPL releases

With these releases of GNAT, you can distribute your programs in binary form under licensing terms of your own choosing; you are not bound by the GPL.

GNAT 3.15p

This is the last public release of GNAT from AdaCore that uses the GNAT Modified General Public License.

GNAT 3.15p has passed the Ada Conformity Assessment Test Suite (ACATS). It was released in October 2002.

The binary distribution from AdaCore also contains an Ada-aware version of the GNU Debugger (GDB), and a graphical front-end to GDB called the GNU Visual Debugger (GVD).

GNAT Pro

GNAT Pro is the professional version of GNAT, offered as a subscription package by AdaCore. The package also includes professional consulting, training and maintenance services. AdaCore can provide custom versions of the compiler for native or cross development. For more information, see http://www.adacore.com/.

GCC

GNAT has been part of GCC since October 2001, and GCC uses the GNAT Modified General Public License for the Ada run-time library. The Free Software Foundation does not distribute binaries, only sources.

Most GNU/Linux distributions and several distributions for other platforms include prebuilt binaries; see below.

For technical reasons, we recommend against using the Ada compilers included in GCC 3.1, 3.2, 3.3 and 4.0. Instead, we recommend using GCC 3.4, 4.1 or later, or one of the releases from AdaCore (3.15p, GPL Edition or Pro).

Since October 2003, AdaCore merge most of their changes from GNAT Pro into GCC during Stage 1; this happens once for each major release. Since GCC 3.4, AdaCore have gradually added support for Ada 2005 as the language standard was finalised.

GCC version 4.4 switched to version 3 of the GNU General Public License and grants a Runtime Library Exception similar in spirit to the GNAT Modified General Public License used in all previous versions. This Runtime Library Exception applies to run-time libraries for all languages, not just Ada.

As of GCC 4.4, released on 2009-04-21:

The GNU Ada Project

The GNU Ada Project provides source and binary packages of various GNAT versions for several operating systems, and, importantly, the scripts used to create the packages. This may be helpful if you plan to port the compiler to another platform or create a cross-compiler. For GNU/Linux users, there are instructions for building your own GNAT compiler at: http://ada.krischik.com/index.php/Articles/CompileGNAT

Both GPL and GMGPL versions of GNAT are available.

A# (A-Sharp, a.k.a. Ada for .NET)

This compiler is historical as it has now been merged into GNAT GPL Edition and GNAT Pro.

A# is a port of Ada to the .NET Platform. A# was originally developed at the Department of Computer Science at the United States Air Force Academy which distribute A# as a service to the Ada community under the terms of the GNU general public license. A# integrates well with Microsoft Visual Studio 2005, AdaGIDE and the RAPID open-source GUI Design tool. As of 2006-06-06:

GNAT for AVR microcontrollers

Rolf Ebert and others provide a version of GNAT configured as a cross-compiler to various AVR microcontrollers, as well as an experimental Ada run-time library suitable for use on the microcontrollers. As of Version 0.4.1 (2006-06-01):

GNATforLEON

The Real-Time Research Group of the Technical University of Madrid (UPM, Universidad Politécnica de Madrid) wrote a Ravenscar-compliant real-time kernel for execution on LEON processors and a modified run-time library. They also provide a GNAT cross-compiler. As of version 2.0.1:

Prebuilt packages as part of larger distributions

Many distributions contain prebuilt binaries of GCC or various public releases of GNAT from AdaCore. Quality varies widely between distributions. The list of distributions below is in alphabetical oder. (Please keep it that way.)

AIDE (for Microsoft Windows)

AIDE — Ada Instant Development Environment is a complete one-click, just-works Ada distribution for Windows, consisting of GNAT, comprehensive documentation, tools and libraries. All are precompiled, and source code is also available. The installation procedure is particularly easy. AIDE is intended for beginners and teachers, but can also be used by advanced users.

Blastwave (for Solaris on SPARC and x86)

Blastwave has binary packages of GCC 3.4.5 and 4.0.2 with Ada support. The package names are gcc3ada and gcc4ada respectively.

Cygwin (for Microsoft Windows)

Cygwin, the Linux-like environment for Windows, also contains a version of the GNAT compiler. The Cygwin version of GNAT is older than the MinGW version and does not support DLLs and Multi-Threading (as of 11.2004).

Debian (GNU/Linux and GNU/kFreeBSD)

There is a Debian Policy for Ada (mirror), which tries to make Debian the best Ada development and deployment platform. The development platform includes the compiler and many libraries, pre-packaged and integrated so as to be easy to use in any program. The deployment platform is the renowned stable distribution, which is suitable for mission-critical workloads and enjoys long life cycles, typically 3 to 4 years. Because Debian is a binary distribution, it is possible to deploy non-free, binary-only programs on it while enjoying all the benefits of a stable platform. Compiler choices are conservative for this reason, and the Policy mandates that all Ada programs and libraries be compiled with the same version of GNAT. This makes it possible to use all libraries in the same program. Debian separates run-time libraries from development packages, so that end users do not have to install the development system just to run a program.

The GNU Ada compiler can be installed on a Debian system with this command:

apt-get install gnat

This will also give you a list of related packages, which are likely to be useful for an Ada programmer.

Debian is unique in that it also allows programmers to use some of GNAT's internal components by means of two libraries:

• libgnatvsn (licensed under GNAT-Modified GPL) and
• libgnatprj (the project manager, licensed under pure GPL).

Debian packages make use of these libraries.

The ADT plugin for Eclipse (see #ObjectAda_from_Aonix) can be used with GNAT as packaged for Debian Etch. Specify "/usr" as the toolchain path.

DJGPP (for MS-DOS)

DJGPP has GNAT as part of their GCC distribution.

DJGPP is a port of a comprehensive collection of GNU utilities to MS-DOS with 32-bit extensions, and is actively supported (as of 1.2005). It includes the whole GCC compiler collection, that now includes Ada. See the DJGPP website for installation instructions.

DJGPP programs run also in a DOS command box in Windows, as well as in native MS-DOS systems.

FreeBSD

FreeBSD's ports collection contains GNAT GPL 2006 Edition (package gnat-2006), GNAT 3.15p, GCC 4.1, 4.2 and 4.3 with support for Ada. The usual way to install a package on FreeBSD is to compile it from source; not all add-ons are compatible with all versions of GNAT provided.

You can also use the Debian packages described above in a jail, thanks to Debian GNU/kFreeBSD.

As of 2008-11-10:

Gentoo GNU/Linux

The GNU Ada compiler can be installed on a Gentoo system using emerge:

 emerge dev-lang/gnat

In contrast to Debian, Gentoo is primarily a source distribution, so many packages are available only in source form, and require the user to recompile them (using emerge).

Also in contrast to Debian, Gentoo supports several versions of GNAT in parallel on the same system. Be careful, because not all add-ons and libraries are available with all versions of GNAT.

GNAT for Macintosh (for Mac OS 9 and X)

GNAT for Macintosh provides both FSF (GMGPL) and AdaCore (GPL) versions of GNAT with Xcode integration and bindings.

Mandriva Linux

The GNU Ada compiler can be installed on a Mandriva system with this command:

urpmi gnat

MinGW (for Microsoft Windows)

MinGW — Minimalist GNU for Windows contains a version of the GNAT compiler.

The following list should help you with the installation. (I may have forgotten something — but this is wiki, just add to the list)

1. Install MinGW-3.1.0-1.exe
   1. extract binutils-2.15.91-20040904-1.tar.gz
   2. extract mingw-runtime-3.5.tar.gz
   3. extract gcc-core-3.4.2-20040916-1.tar.gz
   4. extract gcc-ada-3.4.2-20040916-1.tar.gz
   5. extract gcc-g++-3.4.2-20040916-1.tar.gz (Optional)
   6. extract gcc-g77-3.4.2-20040916-1.tar.gz (Optional)
   7. extract gcc-java-3.4.2-20040916-1.tar.gz (Optional)
   8. extract gcc-objc-3.4.2-20040916-1.tar.gz (Optional)
   9. extract w32api-3.1.tar.gz
2. Install mingw32-make-3.80.0-3.exe (Optional)
3. Install gdb-5.2.1-1.exe (Optional)
4. Install MSYS-1.0.10.exe (Optional)
5. Install msysDTK-1.0.1.exe (Optional)
   1. extract msys-automake-1.8.2.tar.bz2 (Optional)
   2. extract msys-autoconf-2.59.tar.bz2 (Optional)
   3. extract msys-libtool-1.5.tar.bz2 (Optional)

I have made good experience in using D:\MinGW as target directory for all installations and extractions.

Also noteworthy is that the the Windows version for GNAT from Libre is also based on MinGW.

In gcc-3.4.2-release_notes.txt from MinGW site reads: please check that the files in the /lib/gcc/mingw32/3.4.2/adainclude and adalib directories are flagged as read-only. This attribute is necessary to prevent them from being deleted when using gnatclean to clean a project.

So be sure to do this.

SuSE Linux

All versions of SuSE Linux have a GNAT compiler included. SuSE versions 9.2 and higher also contains ASIS, Florist and GLADE libraries. The following two packages are needed:

gnat
gnat-runtime

For 64 bit system you will need the 32 bit compatibility packages as well:

gnat-32bit
gnat-runtime-32bit

Ubuntu

Ubuntu (and derivatives like Kubuntu, Xubuntu...) is a Debian-based Linux distribution, thus the installation process described above can be used. Graphical package managers like Synaptics or Adept can also be employed to select the Ada packages.

ICC from Irvine Compiler Corporation

Irvine Compiler Corporation provides native and cross compilers for various platforms.[5] The compiler and run-time system support development of certified, safety-critical software.

Commercial, proprietary. No-cost evaluation is possible on request. Royalty-free redistribution of the run-time system is allowed.

Janus/Ada 83 and 95 from RR Software

RR Software offers native compilers for MS-DOS, Microsoft Windows and various Unix and Unix-like systems, and a library for Windows GUI programming called CLAW. There are academic, personal and professional editions, as well as support options.

Commercial but relatively cheap; proprietary.

MAXAda from Concurrent

Concurrent offers MAXAda, an Ada 95 compiler for Linux/Xeon and PowerPC platforms, and Ada bindings to POSIX and X/Motif.[6]

Commercial, proprietary.

ObjectAda from Aonix

Aonix offers native and cross compilers for various platforms. They come with an IDE, a debugger and a plug-in for Eclipse.

On Microsoft Windows and GNU/Linux on i386, Aonix offers two pricing models, at the customer's option: either a perpetual license fee with optional support, or just the yearly support fee: For Linux, that's $3000 for a single user or $12,000 for a 5-user service pack. See the full press release.

In addition, they offer "ObjectAda Special Edition": a no-cost evaluation version of ObjectAda that limits the size of programs that can be compiled with it, but is otherwise fully functional, with IDE and debugger. Free registration required.

A recent contribution by Aonix is ADT for Eclipse. The Ada Development Tools add Ada language support to the Eclipse open source development platform. ADT can be used with Aonix compilers, and with GNAT. An open source vendor supported project is outlined for ADT at Eclipse. Codenamed Hibachi and showcased at the Ada Conference UK 2007 and during Ada-Europe 2007, the project has now been officially created.

Commercial, proprietary.

PowerAda from OC Systems

OC Systems offers Ada compilers and bindings to POSIX and X-11:

• PowerAda, an Ada 95 compiler for Linux and AIX,
• LegacyAda/390, an Ada 83 compiler for IBM System 370 and 390 mainframes

Commercial, proprietary.

Rational Apex from IBM Rational

Rational Apex for native and embedded development.

Commercial, proprietary.

SCORE from DDC-I

DDC-I offers its SCORE cross-compilers for embedded development. SCORE stands for Safety-Critical, Object-oriented, Real-time Embedded.

Commercial, proprietary.

XD Ada from SWEP-EDS

XD Ada is an Ada 83 cross-compiler for embedded development. Hosts include Alpha and VAX machines running OpenVMS. Targets include Motorola 68000 and MIL-STD-1750A processors.

Commercial, proprietary.

XGC Ada from XGC Software

XGC compilers are GCC with custom run-time libraries suitable for avionics and space applications. The run-time kernels are very small and do not support exception propagation (i.e. you can handle an exception only in the subprogram that raised it).

Commercial but some versions are also offered as free downloads. Free Software.

Building

Ada programs are usually easier to build than programs written in other languages like C or C++, which frequently require a makefile. This is because an Ada source file already specifies the dependencies of its source unit. See the with keyword for further details.

Building an Ada program is not defined by the Reference Manual, so this process is absolutely dependent on the compiler. Usually the compiler kit includes a make tool which compiles a main program and all its dependencies, and links an executable file.

Building with various compilers

This list is incomplete. You can help Wikibooks by adding the build information for other compilers.

GNAT

With GNAT, you can run this command:

gnat make <your_unit_file>

If the file contains a procedure, gnatmake will generate an executable file with the procedure as main program. Otherwise, e.g. a package, gnatmake will compile the unit and all its dependencies.

GNAT command line

gnatmake can be written as one word gnatmake or two words gnat make. For a full list of gnat commands just type gnat without any command options. The output will look something like this:

GNAT 3.4.3 Copyright 1996-2004 Free Software Foundation, Inc.

List of available commands

GNAT BIND               gnatbind
GNAT CHOP               gnatchop
GNAT CLEAN              gnatclean
GNAT COMPILE            gnatmake -c -f -u
GNAT ELIM               gnatelim
GNAT FIND               gnatfind
GNAT KRUNCH             gnatkr
GNAT LINK               gnatlink
GNAT LIST               gnatls
GNAT MAKE               gnatmake
GNAT NAME               gnatname
GNAT PREPROCESS         gnatprep
GNAT PRETTY             gnatpp
GNAT STUB               gnatstub
GNAT XREF               gnatxref

Commands FIND, LIST, PRETTY, STUB and XREF accept project file switches -vPx, -Pprj and -Xnam=val

For further help on the option just type the command (one word or two words — as you like) without any command options.

GNAT IDE

The GNAT toolchain comes with an IDE called GPS. You need to download and install it separately. The GPS features a graphical user interface.

There are also GNAT plugins for Emacs (Ada Mode), KDevelop and Vim (Ada Mode) available.

Both Emacs and Vim Ada-Mode are maintained by The GNU Ada project.

GNAT with Xcode

Apple's free (gratis) IDE, Xcode, is included with every Macintosh but requires an explicit installation step from DVD-ROM or CD-ROM. It is also downloadable from http://developer.apple.com/. Xcode uses the GNU Compiler Collection and thus supports Ada, GDB, etc... and also includes myriad tools for optimizing code which are unique to the Macintosh platform. However, GNAT must be installed separately as it is (as of 2008) not distributed as part of Xcode. Get the binary and/or sources at http://www.macada.org/, along with numerous tools and bindings including bindings to Apple's Carbon frameworks which allow the development of complete, "real" Mac programs, all in Ada. Be sure to contact the very responsive mailing list if you should encounter any problems.

Rational APEX

Rational APEX is a complete development environment comprising a language sensitive editor, compiler, debugger, coverage analyser, configuration management and much more. You normally work with APEX running a GUI.

APEX has been built for the development of big programs. Therefore the basic entity of APEX is a subsystem, a directory with certain traits recognized by APEX. All Ada compilation units have to reside in subsystems.

You can define an export set, i.e. the set of Ada units visible to other subsystems. However for a subsystem A to gain visibility to another subsystem B, A has to import B. After importing, A sees all units in B's export set. (This is much like the with-clauses, but here visibility means only potential visibility for Ada: units to be actually visible must be mentioned in a with-clause of course; units not in the export set cannot be used in with-clauses of Ada units in external subsystems.)

Normally subsystems should be hierarchically ordered, i.e. form a directed graph. But for special uses, subsystems can also mutually import one another.

For configuration management, a subsystem is decomposed in views, subdirectories of the subsystem. Views hold different development versions of the Ada units. So actually it's not subsystems which import other subsystems, rather subsystem views import views of other subsystems. (Of course, the closure of all imports must be consistent — it cannot be the case that e.g. subsystem (A, view A1) imports subsystems (B, B1) and (C, C1), whereas (B, B1) imports (C, C2)).

A view can be defined to be the development view. Other views then hold releases at different stages.

Each Ada compilation unit has to reside in a file of its own. When compiling an Ada unit, the compiler follows the with-clauses. If a unit is not found within the subsystem holding the compile, the compiler searches the import list (only the direct imports are considered, not the closure).

Units can be taken under version control. In each subsystem, a set of histories can be defined. An Ada unit can be taken under control in a history. If you want to edit it, you first have to check it out — it gets a new version number. After the changes, you can check it in again, i.e. make the changes permanent (or you abandon your changes, i.e. go back to the previous version). You normally check out units in the development view only; check-outs in release views can be forbidden.

You can select which version shall be the active one; normally it is the one latest checked in. You can even switch histories to get different development paths. e.g. different bodies of the same specification for different targets.

ObjectAda

ObjectAda is a set of tools for editing, compiling, navigating and debugging programs written in Ada. There are various editions of ObjectAda. With some editions you compile programs for the same platform and operating systems on which you run the tools. These are called native. With others, you can produce programs for different operating systems and platforms. One possible platform is the Java virtual machine.

These remarks apply to the native Microsoft Windows edition. You can run the translation tools either from the IDE or from the command line.

Whether you prefer to work from the IDE, or from the command line, a little bookkeeping is required. This is done by creating a project. Each project consists of a number of source files, and a number of settings like search paths for additional Ada libraries and other dependences. Each project also has at least one target. Typically, there is a debug target, and a release target. The names of the targets indicate their purpose. At one time you compile for debugging, typically during development, at other times you compile with different settings, for example when the program is ready for release. Some (all commercial?) editions of ObjectAda permit a Java (VM) target.

DEC Ada for VMS

DEC Ada is an Ada 83 compiler for VMS. While “DEC Ada” is probably the name most users know, the compiler is now called “HP Ada”. It had previously been known also by names of "VAX Ada" and "Compaq Ada".

DEC Ada uses a true library management system — so the first thing you need to do is create and activate a library:

ACS Library Create [MyLibrary]
ACS Set Library [MyLibrary]

When creating a library you already set some constraints like support for Long_Float or the available memory size. So carefully read

HELP ACS Library Create *

Then next step is to load your Ada sources into the library:

ACS Load [Source]*.ada

The sources don't need to be perfect at this stage but syntactically correct enough for the compiler to determine the packages declared and analyze the with statements. Dec Ada allows you to have more then one package in one source file and you have any filename convention you like. The purpose of ACS Load is the creation of the dependency tree between the source files.

Next you compile them:

ACS Compile *

Note that compile take the package name and not the filename. The wildcard * means all packages loaded. The compiler automatically determines the right order for the compilation so a make tool is not strictly needed.

Last but not least you link your file into an

ACS Link /Executable=[Executables]Main.exe Main

On large systems you might want to break sources down into several libraries — in which case you also need

ACS Merge /Keep *

to merge the content of the current library with the library higher up the hierarchy. The larger libraries should then be created with:

ACS Library Create /Large

This uses a different directory layout more suitable for large libraries.

DEC Ada IDE

Dec Ada comes without an IDE, however the Ada Mode of the Vim text editor supports DEC Ada.

Compiling our Demo Source

Once you have downloaded our example programs you might wonder how to compile them.

First you need to extract the sources. Use your favorite zip tool to achieve that. On extraction a directory with the same name as the filename is created. Beware: WinZip might also create a directory equaling the filename so Windows users need to be careful using the right option otherwise they end up with wikibook-ada-1_2_0.src\wikibook-ada-1_2_0.

Once you extracted the files you will find all sources in wikibook-ada-1_2_0/Source. You could compile them right there. For your convenience we also provide ready made project files for the following IDEs (If you find a directory for an IDEs not named it might be in the making and not actually work).

GNAT

You will find multi-target GNAT Project files and a multi-make Makefile file in wikibook-ada-2_0_0/GNAT. For i686 Linux and Windows you can compile any demo using:

gnat make -P project_file

You can also open them inside the GPS with

gps -P project_file

For other target platform it is a bit more difficult since you need to tell the project files which target you want to create. The following options can be used:

style ("Debug", "Release")

you can define if you like a debug or release version so you can compare how the options affect size and speed.

os ("Linux", "OS2", "Windows_NT", "VMS") 

choose your operating system. Since there is no Ada 2005 available for OS/2 don't expect all examples to compile.

target ("i686", "x86_64", "AXP")

choose your CPU — "i686" is any form of 32bit Intel or AMD CPU, "x86_64" is an 64 bit Intel or AMD CPU and if you have an "AXP" then you know it.

Remember to type all options as they are shown. To compile a debug version on x86-64 Linux you type:

gnat make -P project_file -Xstyle=Debug -Xos=Linux -Xtarget=x86_64

As said in the beginning there is also a makefile available that will automatically determine the target used. So if you have a GNU make you can save yourself a lot of typing by using:

make project

or even use

make all

to make all examples in debug and release in one go.

Each compile is stored inside it's own directory which is created in the form of wikibook-ada-2_0_0/GNAT/OS-Target-Style. Empty directories are provided inside the archive.

Rational APEX

APEX uses the subsystem and view directory structure, so you will have to create those first and copy the source files into the view. After creating a view using the architecture model of your choice, use the menu option "Compile -> Maintenance -> Import Text Files". In the Import Text Files dialog, add "wikibook-ada-2_0_0/Source/*.ad?" to select the Ada source files from the directory you originally extracted to. Apex uses the file extensions .1.ada for specs and .2.ada for bodies — don't worry, the import text files command will change these automatically.

To link an example, select its main subprogram in the directory viewer and click the link button in the toolbar, or "Compile -> Link" from the menu. Double-click the executable to run it. You can use the shift-key modifier to bypass the link or run dialog.

ObjectAda

ObjectAda command-line

The following describes using the ObjectAda tools for Windows in a console window.

Before you can use the ObjectAda tools from the command line, make sure the PATH environment variable lists the directory containing the ObjectAda tools. Something like

set path=%path%;P:\Programs\Aonix\ObjectAda\bin

A minimal ObjectAda project can have just one source file. like the Hello World program provided in

To build an executable from this source file, follow these steps (assuming the current directory is a fresh one and contains the above mentioned source file):

• Register your source files:

X:\some\directory> adareg hello_world_1.adb

This makes your sources known to the ObjectAda tools. Have a look at the file UNIT.MAP created by adareg in the current directory if you like seeing what is happening under the hood.

• Compile the source file:

X:\some\directory> adacomp hello_world_1.adb
Front end of hello_world_1.adb succeeded with no errors.

• Build the executable program:

X:\some\directory> adabuild hello_world_1
ObjectAda Professional Edition Version 7.2.2: adabuild
   Copyright (c) 1997-2002 Aonix.  All rights reserved.
Linking...
Link of hello completed successfully

Notice that you specify the name of the main unit as argument to adabuild, not the name of the source file. In this case, it is Hello_World_1 as in

procedure Hello_World_1 is

More information about the tools can be found in the user guide Using the command line interface, installed with the ObjectAda tools.

External links

• GNAT Online Documentation:
  • GNAT User's Guide
• DEC Ada:
  • Developing Ada Products on OpenVMS (PDF)
  • DEC Ada — Language Reference Manual (PDF)
  • DEC Ada — Run-Time Reference (PDF)

Control Statements

Conditionals

Conditional clauses are blocks of code that will only execute if a particular expression (the condition) is true.

if-else

The if-else statement is the simplest of the conditional statements. They are also called branches, as when the program arrives at an if statement during its execution, control will "branch" off into one of two or more "directions". An if-else statement is generally in the following form:

if condition then
    statement;
else
    other statement;
end if;

If the original condition is met, then all the code within the first statement is executed. The optional else section specifies an alternative statement that will be executed if the condition is false. Exact syntax will vary between programming languages, but the majority of programming languages (especially procedural and structured languages) will have some form of if-else conditional statement built-in. The if-else statement can usually be extended to the following form:

if condition then
    statement;
elsif condition then
    other statement;
elsif condition then
    other statement;
...
else
    another statement;
end if; 

Only one statement in the entire block will be executed. This statement will be the first one with a condition which evaluates to be true. The concept of an if-else-if structure is easier to understand with the aid of an example:

with Ada.Text_IO;
use  Ada.Text_IO;
...
type Degrees is new Float range -273.15 .. Float'Last;
...
Temperature : Degrees;
...
if Temperature >= 40.0 then
    Put_Line ("Wow!");
    Put_Line ("It's extremely hot");
elsif Temperature >= 30.0 then
    Put_Line ("It's hot");
elsif Temperature >= 20.0 then
    Put_Line ("It's warm");
elsif Temperature >= 10.0 then
    Put_Line ("It's cool");
elsif Temperature >= 0.0 then
    Put_Line ("It's cold");
else
    Put_Line ("It's freezing");
end if; 

Optimizing hints

When this program executes, the computer will check all conditions in order until one of them matches its concept of truth. As soon as this occurs, the program will execute the statement immediately following the condition and continue on, without checking any other condition for truth. For this reason, when you are trying to optimize a program, it is a good idea to sort your if-else conditions in descending probability. This will ensure that in the most common scenarios, the computer has to do less work, as it will most likely only have to check one or two "branches" before it finds the statement which it should execute. However, when writing programs for the first time, try not to think about this too much lest you find yourself undertaking premature optimization.

Having said all that, you should be aware that an optimizing compiler might rearrange your if statement at will when the statement in question is free from side effects. Among other techniques optimizing compilers might even apply jump tables and binary searches.

In Ada, conditional statements with more than one conditional do not use short-circuit evaluation by default. In order to mimic C/C++'s short-circuit evaluation, use and then or or else between the conditions.

case

Often it is necessary to compare one specific variable against several constant expressions. For this kind of conditional expression the case statement exists. For example:

case X is
   when 1 =>

      Walk_The_Dog;

   when 5 =>

      Launch_Nuke;

   when 8 | 10 =>

      Sell_All_Stock;

   when others =>

      Self_Destruct;

end case;

The subtype of X must be a discrete type, i.e. an enumeration or integer type.

In Ada, one advantage of the case statement is that the compiler will check the coverage of the choices, that is, all the values of the subtype of variable X must be present or a default branch when others must specify what to do in the remaining cases.

Unconditionals

Unconditionals let you change the flow of your program without a condition. You should be careful when using unconditionals. Often they make programs difficult to understand. Read Isn't goto evil? for more information.

return

End a function and return to the calling procedure or function.

For procedures:

return;

For functions:

return Value;

goto

Goto transfers control to the statement after the label.

   goto Label;

   Dont_Do_Something;

<<Label>>
   ...

Isn't goto evil?

One often hears that goto is evil and one should avoid using goto. But it is often overlooked that any return which is not the last statement inside a procedure or function is also an unconditional statement — a goto in disguise. There is an important difference though: a return is a forward only use of goto. Exceptions are also a type of goto statement; worse, they need not specify where they are going to!

Therefore if you have functions and procedures with more than one return statement you can just as well use goto. When it comes down to readability the following two samples are almost the same:

procedure Use_Return is
begin
   Do_Something;

   if Test then
      return;
   end if;

   Do_Something_Else;

   return;
end Use_Return;

procedure Use_Goto is
begin
   Do_Something;
  
   if Test then
      goto Exit_Use_Goto;
   end if;

   Do_Something_Else;

<<Exit_Use_Goto>>
   return;
end Use_Goto;

Because the use of a goto needs the declaration of a label, the goto is in fact twice as readable than the use of return. So if readability is your concern and not a strict "don't use goto" programming rule then you should rather use goto than multiple returns. Best, of course, is the structured approach where neither goto nor multiple returns are needed:

procedure Use_If is
begin
   Do_Something;
  
   if not Test then

      Do_Something_Else;

   end if;

   return;
end Use_If;

Loops

Loops allow you to have a set of statements repeated over and over again.

Endless Loop

The endless loop is a loop which never ends and the statements inside are repeated forever. Never is meant as a relative term here — if the computer is switched off then even endless loops will end very abruptly.

Endless_Loop :
   loop

      Do_Something;

   end loop Endless_Loop;

The loop name (in this case, "Endless_Loop") is an optional feature of Ada. Naming loops is nice for readability but not strictly needed. Loop names are useful though if the program should jump out of an inner loop, see below.

Loop with condition at the beginning

This loop has a condition at the beginning. The statements are repeated as long as the condition is met. If the condition is not met at the very beginning then the statements inside the loop are never executed.

While_Loop :
   while X <= 5 loop

      X := Calculate_Something;

   end loop While_Loop;

Loop with condition at the end

This loop has a condition at the end and the statements are repeated until the condition is met. Since the check is at the end the statements are at least executed once.

Until_Loop :
   loop

      X := Calculate_Something;

      exit Until_Loop when X > 5;
   end loop Until_Loop;

Loop with condition in the middle

Sometimes you need to first make a calculation and exit the loop when a certain criterion is met. However when the criterion is not met there is something else to be done. Hence you need a loop where the exit condition is in the middle.

Exit_Loop :
   loop

      X := Calculate_Something;

      exit Exit_Loop when X > 5;

      Do_Something (X);

   end loop  Exit_Loop;

In Ada the exit condition can be combined with any other loop statement as well. You can also have more than one exit statement. You can also exit a named outer loop if you have several loops inside each other.

for loop

Quite often one needs a loop where a specific variable is counted from a given start value up or down to a specific end value. You could use the while loop here — but since this is a very common loop there is an easier syntax available.

For_Loop :
   for I in Integer range 1 .. 10 loop

      Do_Something (I)

   end loop For_Loop;

You don't have to declare both type and range as seen in the example. If you leave out the type then the compiler will determine the type by context and leave out the range then the loop will iterate over every valid value for the type given.

As always with Ada: when "determine by context" gives two or more possible options then an error will be displayed and then you have to name the type to be used. Ada will only do "guess-works" when it is safe to do so.

for loop on arrays

Another very common situation is the need for a loop which iterates over every element of an array. The following sample code shows you how to achieve this:

Array_Loop :
   for I in X'Range loop

      X (I) := Get_Next_Element;

   end loop Array_Loop;

With X being an array. Note: This syntax is mostly used on arrays — hence the name — but will also work with other types when a full iteration is needed.

Unlike other loop counters, the loop counter i, in the for loop statement the value cannot be changed. The following is illegal.

for i in 1 .. 10 loop
   i := i + 1;
end loop;

Also the declaration of the loop counter ceases after the body of the loop.

Working Demo

The following Demo shows how to iterate over every element of an integer type.

with Ada.Text_IO;

procedure Range_1 is
   type Range_Type is range -5 .. 10;

   package T_IO renames Ada.Text_IO;
   package I_IO is new  Ada.Text_IO.Integer_IO (Range_Type);

begin
   for A in Range_Type loop
      I_IO.Put (Item  => A,
                Width => 3,
                Base  => 10);

      if A < Range_Type'Last then
         T_IO.Put (",");
      else
         T_IO.New_Line;
      end if;
   end loop;
end Range_1;

See also

Wikibook

• Ada Programming

Ada Reference Manual

• 5.3 If Statements (Annotated)
• 5.4 Case Statements (Annotated)
• 5.5 Loop Statements (Annotated)
• 5.6 Block Statements (Annotated)
• 5.7 Exit Statements (Annotated)
• 5.8 Goto Statements (Annotated)
• 6.5 Return Statements (Annotated)

Type System

Ada's type system allows the programmer to construct powerful abstractions that represent the real world, and to provide valuable information to the compiler, so that the compiler can find many logic or design errors before they become bugs. It is at the heart of the language, and good Ada programmers learn to use it to great advantage. Four principles govern the type system:

• Strong typing: types are incompatible with one another, so it is not possible to mix apples and oranges. There are, however, ways to convert between types.
• Static typing: type checked while compiling, this allows type errors to be found earlier.
• Abstraction: types represent the real world or the problem at hand; not how the computer represents the data internally. There are ways to specify exactly how a type must be represented at the bit level, but we will defer that discussion to another chapter.
• Name equivalence, as opposed to structural equivalence used in most other languages. Two types are compatible if and only if they have the same name; not if they just happen to have the same size or bit representation. You can thus declare two integer types with the same ranges that are totally incompatible, or two record types with the exact same components, but which are incompatible.

Types are incompatible with one another. However, each type can have any number of subtypes, which are compatible with one another, and with their base type.

Predefined types

There are several predefined types, but most programmers prefer to define their own, application-specific types. Nevertheless, these predefined types are very useful as interfaces between libraries developed independently. The predefined library, obviously, uses these types too.

These types are predefined in the Standard package:

Integer

This type covers at least the range − 2¹⁵ + 1 .. + 2¹⁵ − 1 (RM 3.5.4 (21) (Annotated)). The Standard also defines Natural and Positive subtypes of this type.

Float

There is only a very weak implementation requirement on this type (RM 3.5.7 (14) (Annotated)); most of the time you would define your own floating-point types, and specify your precision and range requirements.

Duration

A fixed point type used for timing. It represents a period of time in seconds (RM A.1 (43) (Annotated)).

Character 

A special form of Enumerations. There are three predefined kinds of character types: 8-bit characters (called Character), 16-bit characters (called Wide_Character), and 32-bit characters (Wide_Wide_Character). Character has been present since the first version of the language (Ada 83), Wide_Character was added in Ada 95, while the type Wide_Wide_Character is available with Ada 2005.

String

Three indefinite array types, of Character, Wide_Character, and Wide_Wide_Character respectively. The standard library contains packages for handling strings in three variants: fixed length (Ada.Strings.Fixed), with varying length below a certain upper bound (Ada.Strings.Bounded), and unbounded length (Ada.Strings.Unbounded). Each of these packages has a Wide_ and a Wide_Wide_ variant.

Boolean 

A Boolean in Ada is an Enumeration of False and True with special semantics.

Packages System and System.Storage_Elements predefine some types which are primarily useful for low-level programming and interfacing to hardware.

System.Address 

An address in memory.

System.Storage_Elements.Storage_Offset 

An offset, which can be added to an address to obtain a new address. You can also subtract one address from another to get the offset between them. Together, Address, Storage_Offset and their associated subprograms provide for address arithmetic.

System.Storage_Elements.Storage_Count 

A subtype of Storage_Offset which cannot be negative, and represents the memory size of a data structure (similar to C's size_t).

System.Storage_Elements.Storage_Element 

In most computers, this is a byte. Formally, it is the smallest unit of memory that has an address.

System.Storage_Elements.Storage_Array 

An array of Storage_Elements without any meaning, useful when doing raw memory access.

The Type Hierarchy

Types are organized hierarchically. A type inherits properties from types above it in the hierarchy. For example, all scalar types (integer, enumeration, modular, fixed-point and floating-point types) have operators "<", ">" and arithmetic operators defined for them, and all discrete types can serve as array indexes.

Ada type hierarchy

Here is a broad overview of each category of types; please follow the links for detailed explanations. Inside parenthesis there are equivalences in C and Pascal for readers familiar with those languages.

Signed Integers (int, INTEGER) 

Signed Integers are defined via the range of values needed.

Unsigned Integers (unsigned, CARDINAL) 

Unsigned Integers are called Modular Types. Apart from being unsigned they also have wrap-around functionality.

Enumerations (enum, char, bool, BOOLEAN) 

Ada Enumeration types are a separate type family.

Floating point (float, double, REAL) 

Floating point types are defined by the digits needed, the relative error bound.

Ordinary and Decimal Fixed Point (DECIMAL) 

Fixed point types are defined by their delta, the absolute error bound.

Arrays ( [ ], ARRAY [ ] OF, STRING ) 

Arrays with both compile-time and run-time determined size are supported.

Record (struct, class, RECORD OF) 

A record is a composite type that groups one or more fields.

Access (*, ^, POINTER TO) 

Ada's Access types may be more than just a simple memory address.

Task & Protected (no equivalence in C or Pascal) 

Task and Protected types allow the control of concurrency

Interfaces (no equivalence in C or Pascal) 

New in Ada 2005, these types are similar to the Java interfaces.

Classification of Types

The types of this hierarchy can be classified as follows.

Specific vs. Class-wide

type T is ...  -- specific
  T'Class      -- class-wide

Operations of specific types are non-dispatching, those on class-wide types are dispatching.

New types can be declared by deriving from specific types; primitive operations are inherited by derivation. You cannot derive from class-wide types.

Constrained vs. Unconstrained

type I is range 1 .. 10;           -- constrained
type AC is array (1 .. 10) of ...  -- constrained

type AU is array (I range <>) of ...          -- unconstrained
type R (X: Discriminant [:= Default]) is ...  -- unconstrained

By giving a constraint to an unconstrained subtype, a subtype or object becomes constrained:

subtype RC is R (Value);  -- constrained subtype of R
OC: R (Value);            -- constrained object of anonymous constrained subtype of R
OU: R;                    -- unconstrained object

Declaring an unconstrained object is only possible if a default value is given in the type declaration above. The language does not specify how such objects are allocated. GNAT allocates the maximum size, so that size changes that might occur with discriminant changes present no problem. Another possibility is implicit dynamic allocation on the heap and deallocation followed be a re-allocation when the size changes.

Definite vs. Indefinite

type I is range 1 .. 10;                     -- definite
type RD (X: Discriminant := Default) is ...  -- definite

type T (<>) is ...                    -- indefinite
type AU is array (I range <>) of ...  -- indefinite
type RI (X: Discriminant) is ...      -- indefinite

Definite subtypes allow the declaration of objects without initial value, since objects of definite subtypes have constraints that are known at creation-time. Object declarations of indefinite subtypes need an initial value to supply a constraint; they are then constrained by the constraint delivered by the initial value.

OT: T  := Expr;                       -- some initial expression (object, function call, etc.)
OA: AU := (3 => 10, 5 => 2, 4 => 4);  -- index range is now 3 .. 5
OR: RI := Expr;                       -- again some initial expression as above

Unconstrained vs. Indefinite

Note that unconstrained subtypes are not necessarily indefinite as can be seen above with RD: it is a definite unconstrained subtype.

Concurrency Types

The Ada language uses types for one more purpose in addition to classifying data + operations. The type system integrates concurrency (threading, parallelism). Programmers will use types for expressing the concurrent threads of control of their programs.

The core pieces of this part of the type system, the task types and the protected types are explained in greater depth in a section on tasking.

Limited Types

Limiting a type means disallowing assignment. The “concurrency types” described above are always limited. Programmers can define their own types to be limited, too, like this:

type T is limited …;

(The ellipsis stands for private, or for a record definition, see the corresponding subsection on this page.) A limited type also doesn't have an equality operator unless the programmer defines one.

You can learn more in the limited types chapter.

Defining new types and subtypes

You can define a new type with the following syntax:

type T is...

followed by the description of the type, as explained in detail in each category of type.

Formally, the above declaration creates a type and its first subtype named T. The type itself, correctly called the "type of T", is anonymous; the RM refers to it as T (in italics), but often speaks sloppily about the type T. But this is an academic consideration; for most purposes, it is sufficient to think of T as a type. For scalar types, there is also a base type called T'Base, which encompasses all values of T.

For signed integer types, the type of T comprises the (complete) set of mathematical integers. The base type is a certain hardware type, symmetric around zero (except for possibly one extra negative value), encompassing all values of T.

As explained above, all types are incompatible; thus:

type Integer_1 is range 1 .. 10;
type Integer_2 is range 1 .. 10;
A : Integer_1 := 8;
B : Integer_2 := A; -- illegal!

is illegal, because Integer_1 and Integer_2 are different and incompatible types. It is this feature which allows the compiler to detect logic errors at compile time, such as adding a file descriptor to a number of bytes, or a length to a weight. The fact that the two types have the same range does not make them compatible: this is name equivalence in action, as opposed to structural equivalence. (Below, we will see how you can convert between incompatible types; there are strict rules for this.)

Creating subtypes

You can also create new subtypes of a given type, which will be compatible with each other, like this:

type Integer_1 is range 1 .. 10;
subtype Integer_2 is Integer_1      range 7 .. 11;  -- bad
subtype Integer_3 is Integer_1'Base range 7 .. 11;  -- OK
A : Integer_1 := 8;
B : Integer_3 := A; -- OK

The declaration of Integer_2 is bad because the constraint 7 .. 11 is not compatible with Integer_1; it raises Contraint_Error at subttype elaboration time.

Integer_1 and Integer_3 are compatible because they are both subtypes of the same type, namely Integer_1'Base.

It is not necessary that the subtype ranges overlap, or be included in one another. The compiler inserts a run-time range check when you assign A to B; if the value of A, at that point, happens to be outside the range of Integer_3, the program raises Constraint_Error.

There are a few predefined subtypes which are very useful:

subtype Natural  is Integer range 0 .. Integer'Last;
subtype Positive is Integer range 1 .. Integer'Last;

Derived types

A derived type is a new, full-blown type created from an existing one. Like any other type, it is incompatible with its parent; however, it inherits the primitive operations defined for the parent type.

type Integer_1 is range 1 .. 10;
type Integer_2 is new Integer_1 range 2 .. 8;
A : Integer_1 := 8;
B : Integer_2 := A; -- illegal!

Here both types are discrete; it is mandatory that the range of the derived type be included in the range of its parent. Contrast this with subtypes. The reason is that the derived type inherits the primitive operations defined for its parent, and these operations assume the range of the parent type. Here is an illustration of this feature:

procedure Derived_Types is

   package Pak is
      type Integer_1 is range 1 .. 10;
      procedure P (I: in Integer_1); -- primitive operation, assumes 1 .. 10
      type Integer_2 is new Integer_1 range 8 .. 10; -- must not break P's assumption
   end Pak;

   package body Pak is
      -- omitted
   end Pak;

   use Pak;
   A: Integer_1 := 4;
   B: Integer_2 := 9;

begin

   P (B); -- OK, call the inherited operation

end Derived_Types;

When we call P (B), the parameter B is converted to Integer_1; this conversion of course passes since the set of acceptable values for the derived type (here, 8 .. 10) must be included in that of the parent type (1 .. 10). Then P is called with the converted parameter.

Consider however a variant of the example above:

procedure Derived_Types is

  package Pak is
    type Integer_1 is range 1 .. 10;
    procedure P (I: in Integer_1; J: out Integer_1);
    type Integer_2 is new Integer_1 range 8 .. 10;
  end Pak;

  package body Pak is
    procedure P (I: in Integer_1; J: out Integer_1) is
    begin
      J := I - 1;
    end P;
  end Pak;

  use Pak;

  A: Integer_1 := 4;  X: Integer_1;
  B: Integer_2 := 8;  Y: Integer_2;

begin

  P (A, X);
  P (B, Y);

end Derived_Types;

When P (B, Y) is called, both parameters are converted to Integer_1. Thus the range check on J (7) in the body of P will pass. However on return parameter Y is converted back to Integer_2 and the range check on Y will of course fail.

With the above in mind, you will see why in the following program Constraint_Error will be called at run time.

procedure Derived_Types is

  package Pak is
    type Integer_1 is range 1 .. 10;
    procedure P (I: in Integer_1; J: out Integer_1);
    type Integer_2 is new Integer_1'Base range 8 .. 12;
  end Pak;

  package body Pak is
    procedure P (I: in Integer_1; J: out Integer_1) is
    begin
      J := I - 1;
    end P;
  end Pak;

  use Pak;

  B: Integer_2 := 11;  Y: Integer_2;

begin

  P (B, Y);

end Derived_Types;

Subtype categories

Ada supports various categories of subtypes which have different abilities. Here is an overview in alphabetical order.

Anonymous subtype

A subtype which does not have a name assigned to it. Such a subtype is created with a variable declaration:

X : String (1 .. 10) := (others => ' ');

Here, (1 .. 10) is the constraint. This variable declaration is equivalent to:

subtype Anonymous_String_Type is String (1 .. 10);

X : Anonymous_String_Type := (others => ' ');

Base type

In Ada, all types are anonymous and only subtypes may be named. For scalar types, there is a special subtype of the anonymous type, called the base type, which is nameable with the 'Base attribute. The base type comprises all values of the first subtype. Some examples:

 type Int is range 0 .. 100;

The base type Int'Base is a hardware type selected by the compiler that comprises the values of Int. Thus it may have the range -2⁷ .. 2⁷-1 or -2¹⁵ .. 2¹⁵-1 or any other such type.

 type Enum  is (A, B, C, D);
 type Short is new Enum range A .. C;

Enum'Base is the same as Enum, but Short'Base also holds the literal D.

Constrained subtype

A subtype of an indefinite subtype that adds constraints. The following example defines a 10 character string sub-type.

 subtype String_10 is String (1 .. 10);

You cannot partially constrain an unconstrained subtype:

 type My_Array is array (Integer range <>, Integer range <>) of Some_Type;

 -- subtype Constr is My_Array (1 .. 10, Integer range <>);  illegal

 subtype Constr is My_Array (1 .. 10, -100 .. 200);

Constraints for all indices must be given, the result is necessarily a definite subtype.

Definite subtype

A definite subtype is a subtype whose size is known at compile-time. All subtypes which are not indefinite subtypes are, by definition, definite subtypes.

Objects of definite subtypes may be declared without additional constraints.

Indefinite subtype

An indefinite subtype is a subtype whose size is not known at compile-time but is dynamically calculated at run-time. An indefinite subtype does not by itself provide enough information to create an object; an additional constraint or explicit initialization expression is necessary in order to calculate the actual size and therefore create the object.

X : String := "This is a string";

X is an object of the indefinite (sub)type String. Its constraint is derived implicitly from its initial value. X may change its value, but not its bounds.

It should be noted that it is not necessary to initialize the object from a literal. You can also use a function. For example:

X : String := Ada.Command_Line.Argument (1);

This statement reads the first command-line argument and assigns it to X.

Named subtype

A subtype which has a name assigned to it. “First subtypes” are created with the keyword type (remember that types are always anonymous, the name in a type declaration is the name of the first subtype), others with the keyword subtype. For example:

type Count_To_Ten is range 1 .. 10;

Count_to_Ten is the first subtype of a suitable integer base type. However, if you would like to use this as an index constraint on String, the following declaration is illegal:

subtype Ten_Characters is String (Count_to_Ten);

This is because String has Positive as index, which is a subtype of Integer (these declarations are taken from package Standard):

subtype Positive is Integer range 1 .. Integer'Last;

type String is (Positive range <>) of Character;

So you have to use the following declarations:

subtype Count_To_Ten is Integer range 1 .. 10;
subtype Ten_Characters is String (Count_to_Ten);

Now Ten_Characters is the name of that subtype of String which is constrained to Count_To_Ten. You see that posing constraints on types versus subtypes has very different effects.

Unconstrained subtype

A subtype of an indefinite subtype that does not add a constraint only introduces a new name for the original subtype.

 subtype My_String is String;

My_String and String are interchangeable.

Qualified expressions

In most cases, the compiler is able to infer the type of an expression; for example:

type Enum is (A, B, C);
E : Enum := A;

Here the compiler knows that A is a value of the type Enum. But consider:

procedure Bad is
   type Enum_1 is (A, B, C);
   procedure P (E : in Enum_1) is... -- omitted
   type Enum_2 is (A, X, Y, Z);
   procedure P (E : in Enum_2) is... -- omitted
begin
   P (A); -- illegal: ambiguous
end Bad;

The compiler cannot choose between the two versions of P; both would be equally valid. To remove the ambiguity, you use a qualified expression:

   P (Enum_1'(A)); -- OK

As seen in the following example, this syntax is often used when creating new objects. If you try to compile the example, it will fail with a compilation error since the compiler will determine that 256 is not in range of Byte.

with Ada.Text_IO;

procedure Convert_Evaluate_As is
   type Byte     is mod 2**8;
   type Byte_Ptr is access Byte;

   package T_IO renames Ada.Text_IO;
   package M_IO is new Ada.Text_IO.Modular_IO (Byte);

   A : constant Byte_Ptr := new Byte'(256);
begin
   T_IO.Put ("A = ");
   M_IO.Put (Item  => A.all,
             Width =>  5,
             Base  => 10);
end Convert_Evaluate_As;

Type conversions

Data does not always come in the format you need them. You must, then, face the task of converting them. Ada as a true multi-purpose language with a special emphasis on "mission critical", "system programming" and "safety" has several conversion techniques. The most difficult part is choosing the right one, so the following list is sorted in order of utility. You should try the first one first; the last technique is a last resort, to be used if all others fails. There are also a few related techniques that you might choose instead of actually converting the data.

Since the most important aspect is not the result of a successful conversion, but how the system will react to an invalid conversion, all examples also demonstrate faulty conversions.

Explicit type conversion

An explicit type conversion looks much like a function call; it does not use the tick (apostrophe, ') like the qualified expression does.

Type_Name (Expression)

The compiler first checks that the conversion is legal, and if it is, it inserts a run-time check at the point of the conversion; hence the name checked conversion. If the conversion fails, the program raises Constraint_Error. Most compilers are very smart and optimise away the constraint checks; so, you need not worry about any performance penalty. Some compilers can also warn that a constraint check will always fail (and optimise the check with an unconditional raise).

Explicit type conversions are legal:

• between any two numeric types
• between any two subtypes of the same type
• between any two types derived from the same type
• between array types under certain conditions (see RM 4.6(24.2/2..24.7/2))
• and nowhere else

(The rules become more complex with class-wide and anonymous access types.)

I: Integer := Integer (10);  -- Unnecessary explicit type conversion
J: Integer := 10;            -- Implicit conversion from universal integer
K: Integer := Integer'(10);  -- Use the value 10 of type Integer: qualified expression
                             -- (qualification not necessary here).

This example illustrates explicit type conversions:

with Ada.Text_IO;

procedure Convert_Checked is
   type Short is range -128 .. +127;
   type Byte  is mod 256;

   package T_IO renames Ada.Text_IO;
   package I_IO is new Ada.Text_IO.Integer_IO (Short);
   package M_IO is new Ada.Text_IO.Modular_IO (Byte);

   A : Short := -1;
   B : Byte;
begin
   B := Byte (A);
   T_IO.Put ("A = ");
   I_IO.Put (Item  =>  A,
             Width =>  5,
             Base  => 10);
   T_IO.Put (", B = ");
   M_IO.Put (Item  =>  B,
             Width =>  5,
             Base  => 10);
end Convert_Checked;

Explicit conversions are possible between any two numeric types: integers, fixed-point and floating-point types. If one of the types involved is a fixed-point or floating-point type, the compiler not only checks for the range constraints, but also performs any loss of precision necessary.

Example 1: the loss of precision causes the procedure to only ever print "0" or "1", since P / 100 is an integer and is always zero or one.

with Ada.Text_IO;
procedure Naive_Explicit_Conversion is
   type Proportion is digits 4 range 0.0 .. 1.0;
   type Percentage is range 0 .. 100;
   function To_Proportion (P : in Percentage) return Proportion is
   begin
      return Proportion (P / 100);
   end To_Proportion;
begin
   Ada.Text_IO.Put_Line (Proportion'Image (To_Proportion (27)));
end Naive_Explicit_Conversion;

Example 2: we use an intermediate floating-point type to guarantee the precision.

with Ada.Text_IO;
procedure Explicit_Conversion is
   type Proportion is digits 4 range 0.0 .. 1.0;
   type Percentage is range 0 .. 100;
   function To_Proportion (P : in Percentage) return Proportion is
      type Prop is digits 4 range 0.0 .. 100.0;
   begin
      return Proportion (Prop (P) / 100.0);
   end To_Proportion;
begin
   Ada.Text_IO.Put_Line (Proportion'Image (To_Proportion (27)));
end Explicit_Conversion;

You might ask why you should convert between two subtypes of the same type. An example will illustrate this.

subtype String_10 is String (1 .. 10);
X: String := "A line long enough to make the example valid";
Slice: constant String := String_10 (X (11 .. 20));

Here, Slice has bounds 1 and 10, whereas X (11 .. 20) has bounds 11 and 20.

Change of Representation

Type conversions can be used for packing and unpacking of records or arrays.

type Unpacked is record
  -- any components
end record;

type Packed is new Unpacked;
for  Packed use record
  -- component clauses for some or for all components
end record;

P: Packed;
U: Unpacked;

P := Packed (U);  -- packs U
U := Unpacked (P);  -- unpacks P

Checked conversion for non-numeric types

The examples above all revolved around conversions between numeric types; it is possible to convert between any two numeric types in this way. But what happens between non-numeric types, e.g. between array types or record types? The answer is two-fold:

• you can convert explicitly between a type and types derived from it, or between types derived from the same type,
• and that's all. No other conversions are possible.

Why would you want to derive a record type from another record type? Because of representation clauses. Here we enter the realm of low-level systems programming, which is not for the faint of heart, nor is it useful for desktop applications. So hold on tight, and let's dive in.

Suppose you have a record type which uses the default, efficient representation. Now you want to write this record to a device, which uses a special record format. This special representation is more compact (uses fewer bits), but is grossly inefficient. You want to have a layered programming interface: the upper layer, intended for applications, uses the efficient representation. The lower layer is a device driver that accesses the hardware directly and uses the inefficient representation.

package Device_Driver is
   type Size_Type is range 0 .. 64;
   type Register is record
      A, B : Boolean;
      Size : Size_Type;
   end record;

   procedure Read (R : out Register);
   procedure Write (R : in Register);
end Device_Driver;

The compiler chooses a default, efficient representation for Register. For example, on a 32-bit machine, it would probably use three 32-bit words, one for A, one for B and one for Size. This efficient representation is good for applications, but at one point we want to convert the entire record to just 8 bits, because that's what our hardware requires.

package body Device_Driver is
   type Hardware_Register is new Register; -- Derived type.
   for Hardware_Register use record
      A at 0 range 0 .. 0;
      B at 0 range 1 .. 1;
      Size at 0 range 2 .. 7;
   end record;

   function Get return Hardware_Register; -- Body omitted
   procedure Put (H : in Hardware_Register); -- Body omitted

   procedure Read (R : out Register) is
      H : Hardware_Register := Get;
   begin
      R := Register (H); -- Explicit conversion.
   end Read;

   procedure Write (R : in Register) is
   begin
      Put (Hardware_Register (R)); -- Explicit conversion.
   end Write;
end Device_Driver;

In the above example, the package body declares a derived type with the inefficient, but compact representation, and converts to and from it.

This illustrates that type conversions can result in a change of representation.

View conversion, in object-oriented programming

Within object-oriented programming you have to distinguish between specific types and class-wide types.

With specific types, only conversions to ancestors are possible and, of course, are checked. During the conversion, you do not "drop" any components that are present in the derived type and not in the parent type; these components are still present, you just don't see them anymore. This is called a view conversion.

There are no conversions to derived types (where would you get the further components from?); extension aggregates have to be used instead.

type Parent_Type is tagged null record;
type Child_Type  is new Parent_Type with null record;

Child_Instance  : Child_Type;

-- View conversion from the child type to the parent type:
Parent_View : Parent_Type := Parent_Type (Child_Instance);

Since, in object-oriented programming, an object of child type is an object of the parent type, no run-time check is necessary.

With class-wide types, conversions to ancestor and child types are possible and are checked as well. These conversions are also view conversions, no data is created or lost.

procedure P (Parent_View : Parent_Type'Class) is
   -- View conversion to the child type:
   One : Child_Type := Child_Type (Parent_View);

   -- View conversion to the class-wide child type:
   Two : Child_Type'Class := Child_Type'Class (Parent_View);

This view conversion involves a run-time check to see if Parent_View is indeed a view of an object of type Child_Type. In the second case, the run-time check accepts objects of type Child_Type but also any type derived from Child_Type.

View renaming

A renaming declaration does not create any new object and performs no conversion; it only gives a new name to something that already exists. Performance is optimal since the renaming is completely done at compile time. We mention it here because it is a common idiom in object oriented programming to rename the result of a view conversion.

type Parent_Type is tagged record
   <components>;
end record;
type Child_Type is new Parent_Type with record
   <further components>;
end record;

Child_Instance : Child_Type;
Parent_View    : Parent_Type'Class renames Parent_Type'Class (Child_Instance);

Now, Parent_View is not a new object, but another name for Child_Instance viewed as the parent, i.e. only the parent components are visible, the further child components are hidden.

Address conversion

Ada's access type is not just a memory location (a thin pointer). Depending on implementation and the access type used, the access might keep additional information (a fat pointer). For example GNAT keeps two memory addresses for each access to an indefinite object — one for the data and one for the constraint informations (Size, First, Last).

If you want to convert an access to a simple memory location you can use the package System.Address_To_Access_Conversions. Note however that an address and a fat pointer cannot be converted reversibly into one another.

The address of an array object is the address of its first component. Thus the bounds get lost in such a conversion.

type My_Array is array (Positive range <>) of Something;
A: My_Array (50 .. 100);

     A'Address = A(A'First)'Address

Unchecked conversion

One of the great criticisms of Pascal was "there is no escape". The reason was that sometimes you have to convert the incompatible. For this purpose, Ada has the generic function Unchecked_Conversion:

generic
   type Source (<>) is limited private;
   type Target (<>) is limited private;
function Ada.Unchecked_Conversion (S : Source) return Target;

Unchecked_Conversion will bit-copy the data and there are absolutely no checks. It is your chore to make sure that the requirements on unchecked conversion as stated in RM 13.9 (Annotated) are fulfilled; if not, the result is implementation dependent and may even lead to abnormal data.

A function call to (an instance of) Unchecked_Conversion will copy the source to the destination. The compiler may also do a conversion in place (every instance has the convention Intrinsic).

To use Unchecked_Conversion you need to instantiate the generic.

In the example below, you can see how this is done. When run, the example it will output "A = -1, B = 255". No error will be reported, but is this the result you expect?

with Ada.Text_IO;
with Ada.Unchecked_Conversion;

procedure Convert_Unchecked is

   type Short is range -128 .. +127;
   type Byte  is mod 256;

   package T_IO renames Ada.Text_IO;
   package I_IO is new Ada.Text_IO.Integer_IO (Short);
   package M_IO is new Ada.Text_IO.Modular_IO (Byte);

   function Convert is new Ada.Unchecked_Conversion (Source => Short,
                                                     Target => Byte);

   A : constant Short := -1;
   B : Byte;

begin

   B := Convert (A);
   T_IO.Put ("A = ");
   I_IO.Put (Item  =>  A,
             Width =>  5,
             Base  => 10);
   T_IO.Put (", B = ");
   M_IO.Put (Item  =>  B,
             Width =>  5,
             Base  => 10);

end Convert_Unchecked;

Overlays

If the copying of the result of Unchecked_Conversion is too much waste in terms of performance, then you can try overlays, i.e. address mappings. By using overlays, both objects share the same memory location. If you assign a value to one, the other changes as well. The syntax is:

for Target'Address use expression;
pragma Import (Ada, Target);

where expression defines the address of the source object.

While overlays might look more elegant than Unchecked_Conversion, you should be aware that they are even more dangerous and have even greater potential for doing something very wrong. For example if Source'Size < Target'Size and you assign a value to Target, you might inadvertently write into memory allocated to a different object.

You have to take care also of implicit initializations of objects of the target type, since they would overwrite the actual value of the source object. The Import pragma with convention Ada can be used to prevent this, since it avoids the implicit initialization, RM B.1 (Annotated).

The example below does the same as the example from "Unchecked Conversion".

with Ada.Text_IO;

procedure Convert_Address_Mapping is
   type Short is range -128 .. +127;
   type Byte  is mod 256;

   package T_IO renames Ada.Text_IO;
   package I_IO is new Ada.Text_IO.Integer_IO (Short);
   package M_IO is new Ada.Text_IO.Modular_IO (Byte);

   A : aliased Short;
   B : aliased Byte;
  
   for B'Address use A'Address;
  pragma Import (Ada, B);
  
begin
   A := -1;
   T_IO.Put ("A = ");
   I_IO.Put (Item  =>  A,
             Width =>  5,
             Base  => 10);
   T_IO.Put (", B = ");
   M_IO.Put (Item  =>  B,
             Width =>  5,
             Base  => 10);
end Convert_Address_Mapping;

Export / Import

Just for the record: There is still another method using the Export and Import pragmas. However, since this method completely undermines Ada's visibility and type concepts even more than overlays, it has no place here in this language introduction and is left to experts.

Elaborated Discussion of Types for Signed Integer Types

As explained before, a type declaration

type T is range 1 .. 10;

declares an anonymous type T and its first subtype T (please note the italization). T encompasses the complete set of mathematical integers. Static expressions and named numbers make use of this fact.

All numeric integer literals are of type Universal_Integer. They are converted to the appropriate specific type where needed. Universal_Integer itself has no operators.

Some examples with static named numbers:

 S1: constant := Integer'Last + Integer'Last;       -- "+" of Integer
 S2: constant := Long_Integer'Last + 1;             -- "+" of Long_Integer
 S3: constant := S1 + S2;                           -- "+" of root_integer
 S4: constant := Integer'Last + Long_Integer'Last;  -- illegal

Static expressions are evaluated at compile-time on the appropriate types with no overflow checks, i.e. mathematically exact (only limited by computer store). The result is then implicitly converted to Universal_Integer.

The literal 1 in S2 is of type Universal_Integer and implicitly converted to Long_Integer.

S3 implicitly converts the summands to root_integer, performs the calculation and converts back to Universal_Integer.

S4 is illegal because it mixes two different types.

root_integer is the anonymous greatest integer type representable by the hardware. It has the range System.Min_Integer .. System.Max_Integer. All integer types are rooted at root_integer, i.e. derived from it. Universal_Integer can be viewed as root_integer'Class.

During run-time, computations of course are performed with range checks and overflow checks on the appropriate subtype. Intermediate results may however exceed the range limits. Thus with I, J, K of the subtype T above, the following code will return the correct result:

I := 10;
J :=  8;
K := (I + J) - 12;
-- I := I + J;  -- range check would fail, leading to Constraint_Error

Real literals are of type Universal_Real, and similar rules as the ones above apply accordingly.

See also

Wikibook

• Ada Programming

Ada Reference Manual

• 3.2.1 Type Declarations (Annotated)
• 3.3 Objects and Named Numbers (Annotated)
• 3.7 Discriminants (Annotated)
• 3.10 Access Types (Annotated)
• 4.9 Static Expressions and Static Subtypes (Annotated)
• 13.9 Unchecked Type Conversions (Annotated)
• 13.3 Operational and Representation Attributes (Annotated)
• Annex K (informative) Language-Defined Attributes (Annotated)

Integer types

A range is a signed integer value which ranges from a First to a last Last. It is defined as

 range First .. Last

When a value is assigned to an object with such a range constraint, the value is checked for validity and Constraint_Error exception is raised when the value is not within First to Last.

When declaring a range type, the corresponding mathematical operators are implicitly declared by the language at the same place.

The compiler is free to choose a suitable underlaying hardware type for this user defined type.

Working demo

The following Demo defines a new range from -5 to 10 and then prints the whole range out.

with Ada.Text_IO;

procedure Range_1 is
   type Range_Type is range -5 .. 10;

   package T_IO renames Ada.Text_IO;
   package I_IO is new Ada.Text_IO.Integer_IO (Range_Type);

begin
   for A in Range_Type loop
      I_IO.Put (
         Item  => A,
         Width => 3,
         Base  => 10);

      if A < Range_Type'Last then
         T_IO.Put (",");
      else
         T_IO.New_Line;
      end if;
   end loop;
end Range_1;

See also

Wikibook

• Ada Programming
• Ada Programming/Types
• Ada Programming/Keywords/range

Ada Reference Manual

• 4.4 Expressions (Annotated)
• 3.5.4 Integer Types (Annotated)

Unsigned integer types

Description

Unsigned integers in Ada have a value range from 0 to some positive number (not necessarily one less than a power of 2). They are defined using the mod keyword because they implement a wrap-around arithmetic.

 mod Modulus

where 'First is 0 and 'Last is Modulus - 1.

Wrap-around arithmetic means that 'Last + 1 = 0 = 'First, and 'First - 1 = 'Last. Additionally to the normal arithmetic operators, bitwise and, or and xor are defined for the type.

The predefined package Interfaces (RM B.2 (Annotated)) presents unsigned integers based on powers of 2

type Unsigned_n is mod 2**n;

for which also shift and rotate operations are defined. The values of n depend on compiler and target architecture.

You can use range to sub-range a modular type:

type Byte is mod 256;
subtype Half_Byte is Byte range 0 .. 127;

But beware: the Modulus of Half_Byte is still 256! Arithmetic with such a type is interesting to say the least.

See also

Wikibook

• Ada Programming
• Ada Programming/Types
• Ada Programming/Keywords/mod

Ada Reference Manual

• 4.4 Expressions (Annotated)
• 3.5.4 Integer Types (Annotated)

Enumerations

An enumeration type is defined as a list of possible values:

 type Primary_Color is (Red, Green, Blue);

Like for numeric types, where e.g. 1 is an integer literal, Red, Green and Blue are called the literals of this type. There are no other values assignable to objects of this type.

Operators and attributes

Apart from equality ("="), the only operators on enumeration types are the ordering operators: "<", "<=", "=", "/=", ">=", ">", where the order relation is given implicitly by the sequence of literals: Each literal has a position, starting with 0 for the first, incremented by one for each successor. This position can be queried via the 'Pos attribute; the inverse is 'Val, which returns the corresponding literal. In our example:

Primary_Color'Pos (Red) = 0
Primary_Color'Val (0)   = Red

There are two other important attributes: Image and Value (don't confuse Val with Value). Image returns the string representation of the value (in capital letters), Value is the inverse:

Primary_Color'Image ( Red ) = "RED"
Primary_Color'Value ("Red") =  Red

These attributes are important for simple IO (there are more elaborate IO facilities in Ada.Text_IO for enumeration types). Note that, since Ada is case-insensitive, the string given to 'Value can be in any case.

Enumeration literals

Literals are overloadable, i.e. you can have another type with the same literals.

type Traffic_Light is (Red, Yellow, Green);

Overload resolution within the context of use of a literal normally resolves which Red is meant. Only if you have an unresolvable overloading conflict, you can qualify with special syntax which Red is meant:

Primary_Color'(Red)

Like many other declarative items, enumeration literals can be renamed. In fact, such a literal is a actually function, so it has to be renamed as such:

function Red return P.Primary_Color renames P.Red;

Here, Primary_Color is assumed to be defined in package P, which is visible at the place of the renaming declaration. Renaming makes Red directly visible without necessity to resort the use-clause.

Note that redeclaration as a function does not affect the staticness of the literal.

Characters as enumeration literals

Rather unique to Ada is the use of character literals as enumeration literals:

 type ABC is ('A', 'B', 'C');

This literal 'A' has nothing in common with the literal 'A' of the predefined type Character (or Wide_Character).

Every type that has at least one character literal is a character type. For every character type, string literals and the concatenation operator "&" are also implicitly defined.

type My_Character is (No_Character, 'a', Literal, 'z');
type My_String is array (Positive range <>) of My_Character;

S: My_String := "aa" & Literal & "za" & 'z';
T: My_String := ('a', 'a', Literal, 'z', 'a', 'z');

In this example, S and T have the same value.

Ada's Character type is defined that way. See Ada Programming/Libraries/Standard.

Booleans as enumeration literals

Also Booleans are defined as eunumeration types:

 type Boolean is (False, True);

There is special semantics implied with this declaration in that objects and expressions of this type can be used as conditions. Note that the literals False and True are not Ada keywords.

Thus it is not sufficient to declare a type with these literals and then hope objects of this type can be used like so:

 type My_Boolean is (False, True);
 Condition: My_Boolean;

 if Condition then  -- wrong, won't compile

If you need your own Booleans (perhaps with special size requirements), you have to derive from the predefined Boolean:

 type My_Boolean is new Boolean;
 Condition: My_Boolean;

 if Condition then  -- OK

Enumeration subtypes

You can use range to subtype an enumeration type:

 subtype Capital_Letter is Character range 'A' .. 'Z';

 type Day_Of_Week is (Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday);
 
 subtype Working_Day is range Monday .. Friday;

See also

Wikibook

• Ada Programming
• Ada Programming/Types
• Ada Programming/Libraries/Standard

Ada Reference Manual

• 3.5.1 Enumeration Types (Annotated)

Floating point types

Description

Wikipedia has related information at Floating point

To define a floating point type, you only have to say how many digits are needed, i.e. you define the relative precision:

digits Num_Digits

If you like, you can declare the minimum range needed as well:

digits Num_Digits range Low .. High

This facility is a great benefit of Ada over (most) other programming languages. In other languages, you just choose between "float" and "long float", and what most people do is:

• choose float if they don't care about accuracy
• otherwise, choose long float, because it is the best you can get

In either case, you don't know what accuracy you get.

In Ada, you specify the accuracy you need, and the compiler will choose an appropriate floating point type with at least the accuracy you asked for. This way, your requirement is guaranteed. Moreover, if the computer has more than two floating point types available, the compiler can make use of all of them.

See also

Wikibook

• Ada Programming
• Ada Programming/Types
• Ada Programming/Types/range
• Ada Programming/Types/delta
• Ada Programming/Types/mod
• Ada Programming/Keywords/digits

Ada Reference Manual

• 3.5.7 Floating Point Types (Annotated)

Fixed point types

Description

Wikipedia has related information at Fixed-point arithmetic

A fixed point type defines a set of values that are evenly spaced with a given absolute precision. In contrast, floating point values are all spaced according to a relative precision.

The absolute precision is given as the delta of the type. There are two kinds of fixed point types, ordinary and decimal.

For Ordinary Fixed Point types, the delta gives a hint to the compiler how to choose the small value if it is not specified: It can be any integer power of two not greater than delta. You may specify the small via an attribute clause to be any value not greater than delta. (If the compiler cannot conform to this small value, it has to reject the declaration.)

For Decimal Fixed Point types, the small is defined to be the delta, which in turn must be an integer power of ten. (Thus you cannot specify the small by an attribute clause.)

For example, if you define a decimal fixed point type with a delta of 0.1, you will be able to accurately store the values 0.1, 1.0, 2.2, 5.7, etc. You will not be able to accurately store the value 0.01. Instead, the value will be rounded down to 0.0.

If the compiler accepts your fixed point type definition, it guarantees that values represented by that type will have at least the degree of accuracy specified (or better). If the compiler cannot support the type definition (e.g. due to limited hardware) then a compile-time error will result.

Ordinary Fixed Point

For an ordinary fixed point, you just define the delta and a range:

delta Delta range Low .. High

The delta can be any real value — for example you may define a circle with one arcsecond resolution with:

delta 1 / (60 * 60) range 0.0 .. 360.0

[There is one rather strange rule about fixed point types: Because of the way they are internally represented, the range might only go up to 'Last - Delta. This is a bit like a circle — there 0° and 360° is also the same.]

It should be noted that in the example above the smallest possible value used is not . The compiler will choose a smaller value which, by default, is an integer power of 2 not greater than the delta. In our example this could be . In most cases this should render better performance but sacrifices precision for it.

If this is not what you wish and precision is indeed more important, you can choose your own small value via the attribute clause 'Small.

type Angle is delta Pi/2.0**31 range -Pi .. Pi;
for Angle'Small use Pi/2.0**31;

As internal representation, you will get a 32 bit signed integer type.

Decimal Fixed Point

You define a decimal fixed point by defining the delta and the number of digits needed:

 delta Delta digits Num_Digits

Delta must be a positive or negative integer power of 10 — otherwise the declaration is illegal.

delta 10.0**(+2) digits 12
delta 10.0**(-2) digits 12

If you like, you can also define the range needed:

delta Delta_Value digits Num_Digits range Low .. High

Differences between Ordinary and Decimal Fixed Point Types

There is an alternative way of declaring a "decimal" fixed point: You declare an ordinary fixed point and use an integer power of 10 as 'Small. The following two declarations are equivalent with respect to the internal representation:

-- decimal fixed point

type Duration is delta 10.0**(-9) digits 9;

-- ordinary fixed point

type Duration is delta 10.0**(-9) range -1.0 .. 1.0;
for Duration'Small use 10.0**(-9);

You might wonder what the difference then is between these two declarations. The answer is:

None with respect to precision, addition, subtraction, multiplication with integer values.

The following is an incomplete list of differences between ordinary and decimal fixed point types.

• Decimal fixed point types are intended to reflect typical COBOL declarations with a given number of digits.

• Truncation is required for decimal, not for ordinary, fixed point in multiplication and division (RM 4.5.5 (21) (Annotated)) and type conversions. Operations on decimal fixed point are fully specified, which is not true for ordinary fixed point.

• The following attributes are only defined for decimal fixed point: T'Digits (RM 3.5.10 (10) (Annotated)) corresponds to the number of decimal digits that are representable; T'Scale (RM 3.5.10 (11) (Annotated), taken from COBOL) indicates the position of the point relative to the rightmost significant digits; T'Round (RM 3.5.10 (12) (Annotated)) can be used to specify rounding on conversion.

• Package Decimal (RM F.2 (Annotated)), which of course applies only to decimal fixed point, defines the decimal Divide generic procedure. If annex F is supported (GNAT does), at least 18 digits must be supported (there is no such rule for fixed point).

• Decimal_IO (RM A.10.1 (73) (Annotated)) has semantics different from Fixed_IO (RM A.10.1 (68) (Annotated)).

• Static expressions must be a multiple of the Small for decimal fixed point.

Conclusion: For normal numeric use, an ordinary fixed point (probably with 'Small defined) should be defined. Only if you are interested in COBOL like use, i.e. well defined deterministic decimal semantics (especially for financial computations, but that might apply to cases other than money) should you take decimal fixed point.

See also

Wikibook

• Ada Programming
• Ada Programming/Types
• Ada Programming/Types/range
• Ada Programming/Types/digits
• Ada Programming/Types/mod
• Ada Programming/Keywords/delta
• Ada Programming/Attributes/'Small

Ada 95 Reference Manual

• 3.5.9 Fixed Point Types (Annotated)

Ada 2005 Reference Manual

• 3.5.9 Fixed Point Types (Annotated)

Arrays

An array is a collection of elements which can be accessed by one or more index values. In Ada any definite type is allowed as element and any discrete type, i.e. Range, Modular or Enumeration, can be used as an index.

Declaring arrays

Ada's arrays are quite powerful and so there are quite a few syntax variations, which are presented below.

Basic syntax

The basic form of an Ada array is:

array (Index_Range) of Element_Type

where Index_Range is a range of values within a discrete index type, and Element_Type is a definite subtype. The array consists of one element of "Element_Type" for each possible value in the given range. If you for example want to count how often a specific letter appears inside a text, you could use:

type Character_Counter is array (Character) of Natural;

As a general advice, do not use Integer as the index range, since most of the time negative indices do not make sense. It is also a good style when using numeric indices, to define them starting in 1 instead of 0, since it is more intuitive for humans and avoids off-by-one errors.

With known subrange

Often you don't need an array of all possible values of the index type. In this case you can subtype your index type to the actually needed range.

subtype Index_Sub_Type is Index_Type range First .. Last

array (Index_Sub_Type) of Element_Type

Since this may involve a lot of typing and you may also run out of useful names for new subtypes, the array declaration allows for a shortcut:

array (Index_Type range First .. Last) of Element_Type

Since First and Last are expressions of Index_Type, a simpler form of the above is:

array (First .. Last) of Element_Type

Note that if First and Last are numeric literals, this implies the index type Integer.

If in the example above the character counter should only count upper case characters and discard all other characters, you can use the following array type:

type Character_Counter is array (Character range 'A' .. 'Z') of Natural;

With unknown subrange

Sometimes the range actually needed is not known until runtime or you need objects of different lengths. In lower level languages like C, you would now have to resort to heap memory. Not with Ada. Here we have the box '<>', which allows us to declare indefinite arrays:

array (Index_Type range <>) of Element_Type

When you declare objects of such a type, the bounds must of course be given and the object is constrained to them.

The predefined type String is such a type. It is defined as

 type String is array (Positive range <>) of Character;

You define objects of such an unconstrained type in several ways (the extrapolation to other arrays than String should be obvious):

 Text : String (10 .. 20);
 Input: String := Read_from_some_file;

(These declarations additionally define anonymous subtypes of String.) In the first example, the range of indices is explicitly given. In the second example, the range is implicitly defined from the initial expression, which here could be via a function reading data from some file. Both objects are constrained to their ranges, i.e. they cannot grow nor shrink.

With aliased elements

If you come from C/C++, you are probably used to the fact that every element of an array has an address. The C/C++ standards actually demands that.

In Ada, this is not true. Consider the following array:

 type Day_Of_Month is range 1 .. 31;
 type Day_Has_Appointment is array (Day_Of_Month) of Boolean;
 pragma Pack (Day_Has_Appointment); 

Since we have packed the array, the compiler will use as little storage as possible. And in most cases this will mean that 8 boolean values will fit into one byte.

So Ada knows about arrays where more then one element shares one address. So what if you need to address each single element. Just not using pragma Pack is not enough. If the CPU has very fast bit access, the compiler might pack the array without being told. You need to tell the compiler that you need to address each element via an access.

 type Day_Of_Month is range 1 .. 31;
 type Day_Has_Appointment is array (Day_Of_Month) of aliased Boolean;

Arrays with more than one dimension

Arrays can have more than one index. Consider the following 2-dimensional array:

  type Character_Display is
     array (Positive range <>, Positive range <>) of Character;

This type permits declaring rectangular arrays of characters. Example:

  Magic_Square: constant Character_Display :=
     (('S', 'A', 'T', 'O', 'R'),
      ('A', 'R', 'E', 'P', 'O'),
      ('T', 'E', 'N', 'E', 'T'),
      ('O', 'P', 'E', 'R', 'A'),
      ('R', 'O', 'T', 'A', 'S'));

Or, stating some index values explicitly,

  Magic_Square: constant Character_Display(1 .. 5, 1 .. 5) :=
     (1 => ('S', 'A', 'T', 'O', 'R'),
      2 => ('A', 'R', 'E', 'P', 'O'),
      3 => ('T', 'E', 'N', 'E', 'T'),
      4 => ('O', 'P', 'E', 'R', 'A'),
      5 => ('R', 'O', 'T', 'A', 'S'));

The index values of the second dimension, those indexing the characters in each row, are in 1 .. 5 here. By choosing a different second range, we could change these to be in 11 .. 15:

  Magic_Square: constant Character_Display(1 .. 5, 11 .. 15) :=
     (1 => ('S', 'A', 'T', 'O', 'R'),
       ...