C++ Programming/Code/IO/Streams

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[edit] Streams

Input and output are essential for any computer software, as these are the only means by which the program can communicate with the user. The simplest form of input/output is pure textual, i.e. the application displays in console form, using simple ASCII characters to prompt the user for inputs, which are supplied using the keyboard.

// 'Hello World!' program 
 
#include <iostream>
 
int main()
{
  std::cout << "Hello World!" << std::endl;
  return 0;
}

demonstrates the use of the std::cout stream, known as the standard output stream.

IOStreams are part of the C++ Standard Library concept we saw early not to be confused with the Standard Template Library (STL) that we have yet to introduce.

A stream is a type of object from which we can take values, or to which we can pass values. This is done transparently in terms of the underlying code.

[edit] Stream classes

Input and output is critical to computers. Every program you write will handle i/o in some form to communicate with the user. As this is a very common operation, programming languages like C++ are designed to make i/o as powerful yet painless as possible.

There are many ways for a program to gain input and output, including

• File i/o, that is, reading and writing to files
• Console i/o, reading and writing to a console window, such as a terminal in UNIX-based operating systems or a DOS prompt in Windows.
• Network i/o, reading and writing from a network device
• String i/o, reading and writing treating a string as if it were the input or output device

While these may seem unrelated, they work very similarly. In fact, operating systems that follow the POSIX specification deal with files, devices, network sockets, consoles, and many other things all with one type of handle, a file descriptor. However, low-level interfaces provided by the operating system tend to be difficult to use, so C++, like other languages, provide an abstraction to make programming easier. This abstraction is the stream.

Almost all input and output one ever does can be modeled very effectively as a stream. Having one common model means that one only has to learn it once. If you understand streams, you know the basics of how to output to files, the screen, sockets, pipes, and anything else that may come up.

A stream is an object that allows one to push data in or out of a medium, in order. Usually a stream can only output or can only input. It is possible to have a stream that does both, but this is rare. One can think of a stream as a car driving along a one-way street of information. An output stream can insert data and move on. It (usually) cannot go back and adjust something it has already written. Similarly, an input stream can read the next bit of data and then wait for the one that comes after it. It does not skip data or rewind and see what it had read 5 minutes ago.

The semantics of what a stream's read and write operations do depend on the type of stream. In the case of a file, an input file stream reads the file's contents in order without rewinding, and an output file stream writes to the file in order. For a console stream, output means displaying text, and input means getting input from the user via the console. If the user has not inputted anything, then the program blocks, or waits, for the user to enter in something.

[edit] Standard input, output, and error

The most common streams one uses are cout, cin, and cerr (pronounced "c out", "c in", and "c err(or)", respectively). They are defined in the header <iostream>. Usually, these streams read and write from a console or terminal. In UNIX-based operating systems, such as Linux and Mac OS X, the user can redirect them to other files, or even other programs, for logging or other purposes. They are analogous to stdout, stdin, and stderr found in C. cout is used for generic output, cin is used for input, and cerr is used for printing errors. (cerr typically goes to the same place as cout, unless one or both is redirected, but it is not buffered and allows the user to fine-tune which parts of the program's output is redirected where.)

The standard syntax for outputting to a stream, in this case, cout, is

cout << some_data << some_more_data;

Example

#include <iostream>
 
using namespace std;
 
int main()
{
  int a = 1;
  cout << "Hello world! " << a << '\n';
 
  return 0;
}

Result of Execution

Hello world! 1

To add a line break, send a newline character, \n or use std::endl, which writes a newline and flushes the stream's buffer.

Example

#include <iostream>
#include <ostream>
 
using namespace std;
 
int main()
{
  int a = 1;
  char x = 13;
  cout << "Hello world!" << "\n" << a << endl << x << endl;
 
  return 0;
}

Execution

Hello world!
1

It is always a good idea to end your output with a blank line, so as to not mess up with user's terminals.

[edit] Files

With cout and cin, we can do basic communication with the user. For more complex io, we would like to read to and write from files. This is done with a file stream, defined in the header <fstream>. ofstream is an output file stream, and ifstream is an input file stream.

To open a file, one can either call open on the file stream or, more commonly, use the constructor. One can also supply an open mode to further control the file stream. Open modes include

• ios::app Leaves the file's original contents and appends new data to the end.
• ios::out Outputs new data in the file, removing the old contents. (default for ofstream)
• ios::in Reads data from the file. (default for ifstream)

Example

// open a file called Test.txt and write "HELLO, HOW ARE YOU?" to it
#include <fstream>
 
using namespace std;
 
int main()
{
  ofstream file1;
 
  file1.open("file1.txt", ios::app);
  file1 << "This data will be appended to the file file1.txt\n";
  file1.close();
 
  ofstream file2("file2.txt");
  file2 << "This data will replace the contents of file2.txt\n";
 
  return 0;
}

The call to close() can be omitted if you do not care about the return value (whether it succeeded); the destructors will call close when the object goes out of scope.

If an operation (e.g. opening a file) was unsuccessful, a flag is set in the stream object. You can check the flags' status using the bad() or fail() member functions, which return a boolean value. The stream object doesn't throw any exceptions in such a situation; hence manual status check is required. See reference for details on bad() and fail().

[edit] Manipulators

A manipulator is a function that can be passed as an argument to a stream in different circumstances. For example, the manipulator 'hex' will cause the stream object to format subsequent integer input to the stream in hexadecimal instead of decimal. Likewise, 'oct' results in integers displaying in octal, and 'dec' reverts back to decimal.

Example

#include <iostream>
using namespace std;
 
int main()
{
  cout << dec << 16 << ' ' << 10 << endl;
  cout << oct << 16 << ' ' << 10 << endl;
  cout << hex << 16 << ' ' << 10 << endl;
 
  return 0;
}

Execution

16 10
20 12
10 a

There are many manipulators which can be used in conjunction with streams to simplify the formatting of input. For example, 'setw()' sets the field width of the data item next displayed. Used in conjunction with 'left' and 'right'(which set the justification of the data), 'setw' can easily be used to create columns of data.

Example

#include <iostream>
#include <iomanip>
using namespace std;
 
int main()
{
	cout << setw(10) << right << 90 << setw(8) << "Help!" << endl;
	cout << setw(10) << left << 45 << setw(8) << "Hi!" << endl;
 
	return 0;
}

Execution

        90   Help!
45        Hi!

The data in the top row display at the right of the columns created by 'setw', while in the next row, the data is left justified in the column. Please note the inclusion of a new library 'iomanip'. Most formatting manipulators require this library.

Here are some other manipulators and their uses:

[edit] Buffers

Most stream objects, including 'cout' and 'cin', have an area in memory where the information they are transferring sits until it is asked for. This is called a 'buffer'. Understanding the function of buffers is essential to mastering streams and their use.

Example

#include <iostream>
using namespace std;
 
int main()
{
  int num1, num2;
  cin >> num1;
  cin >> num2;
 
  cout << "Number1: " << num1 << endl
       << "Number2: " << num2 << endl;
 
  return 0;
}

Execution 1

>74
>27
Number1: 74
Number2: 27

The inputs are given separately, with a hard return between them. '>' denotes user input.

Execution 2

>74 27
Number1: 74
Number2: 27

The inputs are entered on the same line. They both go into the 'cin' stream buffer, where they are stored until needed. As 'cin' statements are executed, the contents of the buffer are read into the appropriate variables.

Execution 3

>74 27 56
Number1: 74
Number2: 27

In this example, 'cin' received more input than it asked for. The third number it read in, 56, was never inserted into a variable. It would have stayed in the buffer until 'cin' was called again. The use of buffers can explain many strange behaviors that streams can exhibit.

Example

#include <iostream>
using namespace std;
 
int main()
{
  int num1, num2, num3;
  cin >> num1 >> num2;
 
  cout << "Number1: " << num1 << endl
    << "Number2: " << num2 << endl;
 
  cin >> num3;
 
  cout << "Number3: " << num3 << endl;
 
  return 0;
}

Execution

>45 89 37
Number1: 45
Number2: 89
Number3: 37

Notice how all three numbers were entered at the same time in one line, but the stream only pulled them out of the buffer when they were asked for. This can cause unexpected output, since the user might accidentally put an extra space into his input. A well written program will test for this type of unexpected input and handle it gracefully.

[edit] "safe bool" idiom

or Why define an operator void*() cast operator rather than an operator bool()?

This is so that we cannot write things like:

int foo = std::cin;

or, more importantly,

int bah;
std::cin << bah;    // observe: << instead of >>

by mistake. However, it is not perfect, as it allows other mistakes such as

delete std::cin;

though fortunately such errors are less likely, as delete should be used carefully in any case.

The state of the art would have us instead define a private nested class dummy within std::ios, and return a pointer-to-member-function of dummy -- hence allowing implicit conversion from that to bool, but not allowing many other operations. This is sometimes referred to as the "safe bool" idiom, and is motivated by the fact that C++'s bool type has implicit conversions both to and from int as a result of the standardization process.

[edit] Output

As seen in the "Hello World!" program, we direct the output to std::cout. This means that it is a member of the standard library. For now, don't worry about what this means; we will cover the library and namespaces in later chapters.

What you do need to remember is that, in order to use the output stream, you must include a reference to the standard IO library, as shown here:

#include <iostream>

This opens up a number of streams, functions and other programming devices which we can now use. For this section, we are interested in two of these; std::cout and std::endl.

Once we have referenced the standard IO library, we can use the output stream very simply. To use a stream, give its name, then pipe something in or out of it, as shown:

std::cout << "Hello, world!";

The << operator feeds everything to the right of it into the stream. We have essentially fed a text object into the stream. That's as far as our work goes; the stream now decides what to do with that object. In the case of the output stream, it's printed on-screen.

We're not limited to only sending a single object type to the stream, nor indeed are we limited to one object a time. Consider the examples below:

std::cout << "Hello, " << userName << std::endl;
std::cout << "The answer to life, the universe and everything is " << 42 << std::endl;

As can be seen, we feed in various values, separated by a pipe character. The result comes out something like:

Hello, Joe
The answer to life, the universe and everything is 42

(The name will of course vary; we will discuss variables a little later.)

You will have noticed the use of std::endl throughout some of the examples so far. This is the newline constant. It is a member of the standard IO library, and comes "free" when we instantiate that in order to use the output stream. When the output stream receives this constant, it starts a new line in the console.

And of course, we're not limited to sending only ONE newline, either:

std::cout << "Hello, " << userName << std::endl << std::endl;
std::cout << "How old are you?";

Which produces something like:

Hello, Joe

How old are you?

[edit] Input

What would be the use of an application that only ever outputted information, but didn't care about what its users wanted? Minimal to none. Fortunately, inputting is as easy as outputting when you're using the stream.

The standard input stream is called std::cin and is used very similarly to the output stream. Once again, we instantiate the standard IO library:

#include <iostream>

This gives us access to std::cin (and the rest of that class). Now, we give the name of the stream as usual, and pipe output from it into a variable. A number of things have to happen here, demonstrated in the example below:

#include <iostream>
int main(int argc, char argv[]) {
  int a;
  std::cout << "Hello! How old are you? ";
  std::cin >> a;
  std::cout << "You're really " << a << " years old?" << std::endl;
 
  return 0;
}

We instantiate the standard IO library as usual, and call our main function in the normal way. Now we need to consider where the user's input goes. This calls for a variable (discussed in a later chapter) which we declare as being called a.

Next, we send some output, asking the user for their age. The real input happens now; everything the user types until they hit Enter is going to be stored in the input stream. We pull this out of the input stream and save it in our variable.

Finally, we output the user's age, piping the contents of our variable into the output stream.

Note: You will notice that if anything other than a whole number is entered, the program will crash. This is due to the way in which we set up our variable. Don't worry about this for now; we will cover variables later on.

[edit] Text input until EOF/error/invalid input

Input from the stream infile to a variable data until one of the following:

• EOF reached on infile.
• An error occurs while reading from infile (e.g., connection closed while reading from a remote file).
• The input item is invalid, e.g. non-numeric characters, when data is of type int.

#include <iostream>
 
// ...
 
while (infile >> data)
{
    // manipulate data here
}

Note that the following is not correct:

#include <iostream>
 
// ...
 
while (!infile.eof())
{
    infile >> data; // wrong!
    // manipulate data here
}

This will cause the last item in the input file to be processed twice, because eof() does not return true until input fails due to EOF.

[edit] Making user-created classes compatible with the stream library

It is often useful to have your own classes' instances compatible with the stream framework. For instance, if you defined the class Foo like this:

 class Foo
 {
 public:
 
	Foo() : x(1), y(2)
	{
	}
 
	int x, y;
 };

You will not be able to pass its instance to cout directly using the '<<' operator, because it is not defined for these two objects (Foo and ostream). What needs to be done is to define this operator and thus bind the user-defined class with the stream class.

 ostream& operator<<(ostream& output, Foo& arg)
 {
	output << arg.x << "," << arg.y;
	return output;
 }

Now this is possible:

 Foo my_object;
 cout << "my_object's values are: " << my_object << endl;

The operator function needs to have 'ostream&' as its return type, so chaining output works as usual between the stream and objects of type Foo:

 Foo my1, my2, my3;
 cout << my1 << my2 << my3;

This is because (cout << my1) is of type ostream&, so the next argument (my2) can be appended to it in the same expression, which again gives an ostream& so my3 can be appended and so on.

If you decided to restrict access to the member variables x and y (which is probably a good idea) within the class Foo, i.e.:

 class Foo
 {
 public:
 
	Foo() : x(1), y(2)
	{
	}
 
 private:
	int x, y;
 };

you will have trouble, because the global operator<< function doesn't have access to the private variables of its second argument. There are two possible solutions to this problem:

1. Within the class Foo, declare the operator<< function as the classes' friend which grants it access to private members, i.e. add the following line to the class declaration:

 
 friend ostream& operator<<(ostream& output, Foo& arg);

Then define the operator<< function as you normally would (note that the declared function is not a member of Foo, just its friend, so don't try defining it as Foo::operator<<).

2. Add public-available functions for accessing the member variables and make the operator<< function use these instead:

 class Foo
 {
 public:
 
	Foo() : x(1), y(2)
	{
	}
 
	int get_x()
	{
		return x;
	}
 
	int get_y()
	{
		return y;
	}
 
 private:
	int x, y;
 };
 
 ostream& operator<<(ostream& output, Foo& arg)
 {
	output << arg.get_x() << "," << arg.get_y();
	return output;
 }