Ada Programming/Types/Enumeration

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Ada. Time-tested, safe and secure.
Ada. Time-tested, safe and secure.

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

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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

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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:


Like many other declarative items, enumeration literals can be renamed. In fact, such a literal is actually a 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

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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

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Also Booleans are defined as enumeration 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

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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 Day_Of_Week range Monday .. Friday;

Using enumerations

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Enumeration types being scalar subtypes, type attributes such as First and Succ will allow stepping through a subsequence of the values.

     case Day_Of_Week'First is
        when Sunday =>
           ISO (False);
        when Day_Of_Week'Succ (Sunday) =>
           ISO (True);
        when Tuesday .. Saturday =>
           raise Program_Error;
     end case;

A loop will automatically step through the values of the subtype's range. Filtering week days to include only working days with an even position number:

     for Day in Working_Day loop
        if Day_Of_Week'Pos (Day) mod 2 = 0 then
        end if;
     end loop;

Enumeration types can be used as array index subtypes, yielding a table feature:

  type Officer_ID is range 0 .. 50;
  type Schedule is array (Working_Day) of Officer_ID;

See also

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Ada Reference Manual

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