Types

Scalar Types

In general terms, scalar types are the most basic types that we can get. As we know, we can classify them as follows:

Category

Discrete

Numeric

Enumeration

Yes

No

Integer

Yes

Yes

Real

No

Yes

Many attributes exist for scalar types. For example, we can use the Image and Value attributes to convert between a given type and a string type. The following table presents the main attributes for scalar types:

Category

Attribute

Returned value

Ranges

First

First value of the discrete subtype's range.

Last

Last value of the discrete subtype's range.

Range

Range of the discrete subtype (corresponds to Subtype'First .. Subtype'Last).

Iterators

Pred

Predecessor of the input value.

Succ

Successor of the input value.

Comparison

Min

Minimum of two values.

Max

Maximum of two values.

String conversion

Image

String representation of the input value.

Value

Value of a subtype based on input string.

We already discussed some of these attributes in the Introduction to Ada course (in the sections about range and related attributes and image attribute). In this section, we'll discuss some aspects that have been left out of the previous course.

In the Ada Reference Manual

Ranges

We've seen that the First and Last attributes can be used with discrete types. Those attributes are also available for real types. Here's an example using the Float type and a subtype of it:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;

procedure Show_First_Last_Real is
   subtype Norm is Float range 0.0 .. 1.0;
begin
   Put_Line ("Float'First: " & Float'First'Image);
   Put_Line ("Float'Last:  " & Float'Last'Image);
   Put_Line ("Norm'First:  " & Norm'First'Image);
   Put_Line ("Norm'Last:   " & Norm'Last'Image);
end Show_First_Last_Real;

This program displays the first and last values of both the Float type and the Norm subtype. In the case of the Float type, we see the full range, while for the Norm subtype, we get the values we used in the declaration of the subtype (i.e. 0.0 and 1.0).

Predecessor and Successor

We can use the Pred and Succ attributes to get the predecessor and successor of a specific value. For discrete types, this is simply the next discrete value. For example, Pred (2) is 1 and Succ (2) is 3. Let's look at a complete source-code example:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;

procedure Show_Succ_Pred_Discrete is
   type State is (Idle, Started,
                  Processing, Stopped);

   Machine_State : constant State := Started;

   I : constant Integer := 2;
begin
   Put_Line ("State                     : "
             & Machine_State'Image);
   Put_Line ("State'Pred (Machine_State): "
             & State'Pred (Machine_State)'Image);
   Put_Line ("State'Succ (Machine_State): "
             & State'Succ (Machine_State)'Image);
   Put_Line ("----------");

   Put_Line ("I               : "
             & I'Image);
   Put_Line ("Integer'Pred (I): "
             & Integer'Pred (I)'Image);
   Put_Line ("Integer'Succ (I): "
             & Integer'Succ (I)'Image);
end Show_Succ_Pred_Discrete;

In this example, we use the Pred and Succ attributes for a variable of enumeration type (State) and a variable of Integer type.

We can also use the Pred and Succ attributes with real types. In this case, however, the value we get depends on the actual type we're using:

  • for fixed-point types, the value is calculated using the smallest value (Small), which is derived from the declaration of the fixed-point type;

  • for floating-point types, the value used in the calculation depends on representation constraints of the actual target machine.

Let's look at this example with a decimal type (Decimal) and a floating-point type (My_Float):

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;

procedure Show_Succ_Pred_Real is
   subtype My_Float is
     Float range 0.0 .. 0.5;

   type Decimal is
     delta 0.1 digits 2
     range 0.0 .. 0.5;

   D : Decimal;
   N : My_Float;
begin
   Put_Line ("---- DECIMAL -----");
   Put_Line ("Small: " & Decimal'Small'Image);
   Put_Line ("----- Succ -------");
   D := Decimal'First;
   loop
      Put_Line (D'Image);
      D := Decimal'Succ (D);

      exit when D = Decimal'Last;
   end loop;
   Put_Line ("----- Pred -------");

   D := Decimal'Last;
   loop
      Put_Line (D'Image);
      D := Decimal'Pred (D);

      exit when D = Decimal'First;
   end loop;
   Put_Line ("==================");

   Put_Line ("---- MY_FLOAT ----");
   Put_Line ("----- Succ -------");
   N := My_Float'First;
   for I in 1 .. 5 loop
      Put_Line (N'Image);
      N := My_Float'Succ (N);
   end loop;
   Put_Line ("----- Pred -------");

   for I in 1 .. 5 loop
      Put_Line (N'Image);
      N := My_Float'Pred (N);
   end loop;
end Show_Succ_Pred_Real;

As the output of the program indicates, the smallest value (see Decimal'Small in the example) is used to calculate the previous and next values of Decimal type.

In the case of the My_Float type, the difference between the current and the previous or next values is 1.40130E-45 (or 2-149) on a standard PC.

Scalar To String Conversion

We've seen that we can use the Image and Value attributes to perform conversions between values of a given subtype and a string:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;

procedure Show_Image_Value_Attr is
   I : constant Integer := Integer'Value ("42");
begin
   Put_Line (I'Image);
end Show_Image_Value_Attr;

The Image and Value attributes are used for the String type specifically. In addition to them, there are also attributes for different string types — namely Wide_String and Wide_Wide_String. This is the complete list of available attributes:

Conversion type

Attribute

String type

Conversion to string

Image

String

Wide_Image

Wide_String

Wide_Wide_Image

Wide_Wide_String

Conversion to subtype

Value

String

Wide_Value

Wide_String

Wide_Wide_Value

Wide_Wide_String

We discuss more about Wide_String and Wide_Wide_String in another section.

Width attribute

When converting a value to a string by using the Image attribute, we get a string with variable width. We can assess the maximum width of that string for a specific subtype by using the Width attribute. For example, Integer'Width gives us the maximum width returned by the Image attribute when converting a value of Integer type to a string of String type.

This attribute is useful when we're using bounded strings in our code to store the string returned by the Image attribute. For example:

    
    
    
        
with Ada.Text_IO;         use Ada.Text_IO;
with Ada.Strings;         use Ada.Strings;
with Ada.Strings.Bounded;

procedure Show_Width_Attr is
   package B_Str is new
     Ada.Strings.Bounded.Generic_Bounded_Length
       (Max => Integer'Width);
   use B_Str;

   Str_I : Bounded_String;

   I : constant Integer := 42;
   J : constant Integer := 103;
begin
   Str_I := To_Bounded_String (I'Image);
   Put_Line ("Value:         "
             & To_String (Str_I));
   Put_Line ("String Length: "
             & Length (Str_I)'Image);
   Put_Line ("----");

   Str_I := To_Bounded_String (J'Image);
   Put_Line ("Value:         "
             & To_String (Str_I));
   Put_Line ("String Length: "
             & Length (Str_I)'Image);
end Show_Width_Attr;

In this example, we're storing the string returned by Image in the Str_I variable of Bounded_String type.

Similar to the Image and Value attributes, the Width attribute is also available for string types other than String. In fact, we can use:

  • the Wide_Width attribute for strings returned by Wide_Image; and

  • the Wide_Wide_Width attribute for strings returned by Wide_Wide_Image.

Base

The Base attribute gives us the unconstrained underlying hardware representation selected for a given numeric type. As an example, let's say we declared a subtype of the Integer type named One_To_Ten:

    
    
    
        
package My_Integers is

   subtype One_To_Ten is Integer
     range 1 .. 10;

end My_Integers;

If we then use the Base attribute — by writing One_To_Ten'Base —, we're actually referring to the unconstrained underlying hardware representation selected for One_To_Ten. As One_To_Ten is a subtype of the Integer type, this also means that One_To_Ten'Base is equivalent to Integer'Base, i.e. they refer to the same base type. (This base type is the underlying hardware type representing the Integer type — but is not the Integer type itself.)

For further reading...

The Ada standard defines that the minimum range of the Integer type is -2**15 + 1 .. 2**15 - 1. In modern 64-bit systems — where wider types such as Long_Integer are defined — the range is at least -2**31 + 1 .. 2**31 - 1. Therefore, we could think of the Integer type as having the following declaration:

type Integer is
  range -2 ** 31 .. 2 ** 31 - 1;

However, even though Integer is a predefined Ada type, it's actually a subtype of an anonymous type. That anonymous "type" is the hardware's representation for the numeric type as chosen by the compiler based on the requested range (for the signed integer types) or digits of precision (for floating-point types). In other words, these types are actually subtypes of something that does not have a specific name in Ada, and that is not constrained.

In effect,

type Integer is
  range -2 ** 31 .. 2 ** 31 - 1;

is really as if we said this:

subtype Integer is
  Some_Hardware_Type_With_Sufficient_Range
  range -2 ** 31 .. 2 ** 31 - 1;

Since the Some_Hardware_Type_With_Sufficient_Range type is anonymous and we therefore cannot refer to it in the code, we just say that Integer is a type rather than a subtype.

Let's focus on signed integers — as the other numerics work the same way. When we declare a signed integer type, we have to specify the required range, statically. If the compiler cannot find a hardware-defined or supported signed integer type with at least the range requested, the compilation is rejected. For example, in current architectures, the code below most likely won't compile:

    
    
    
        
package Int_Def is

   type Too_Big_To_Fail is
     range -2 ** 255 .. 2 ** 255 - 1;

end Int_Def;

Otherwise, the compiler maps the named Ada type to the hardware "type", presumably choosing the smallest one that supports the requested range. (That's why the range has to be static in the source code, unlike for explicit subtypes.)

The following example shows how the Base attribute affects the bounds of a variable:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;
with My_Integers; use My_Integers;

procedure Show_Base is
   C : constant One_To_Ten := One_To_Ten'Last;
begin
   Using_Constrained_Subtype : declare
      V : One_To_Ten := C;
   begin
      Put_Line
        ("Increasing value for One_To_Ten...");

      V := One_To_Ten'Succ (V);
   exception
      when others =>
         Put_Line ("Exception raised!");
   end Using_Constrained_Subtype;

   Using_Base : declare
      V : One_To_Ten'Base := C;
   begin
      Put_Line
      ("Increasing value for One_To_Ten'Base...");

      V := One_To_Ten'Succ (V);
   exception
      when others =>
         Put_Line ("Exception raised!");
   end Using_Base;

   Put_Line ("One_To_Ten'Last: "
             & One_To_Ten'Last'Image);
   Put_Line ("One_To_Ten'Base'Last: "
             & One_To_Ten'Base'Last'Image);
end Show_Base;

In the first block of the example (Using_Constrained_Subtype), we're asking for the next value after the last value of a range — in this case, One_To_Ten'Succ (One_To_Ten'Last). As expected, since the last value of the range doesn't have a successor, a constraint exception is raised.

In the Using_Base block, we're declaring a variable V of One_To_Ten'Base subtype. In this case, the next value exists — because the condition One_To_Ten'Last + 1 <= One_To_Ten'Base'Last is true —, so we can use the Succ attribute without having an exception being raised.

In the following example, we adjust the result of additions and subtractions to avoid constraint errors:

    
    
    
        
package My_Integers is

   subtype One_To_Ten is Integer range 1 .. 10;

   function Sat_Add (V1, V2 : One_To_Ten'Base)
                     return One_To_Ten;

   function Sat_Sub (V1, V2 : One_To_Ten'Base)
                     return One_To_Ten;

end My_Integers;

    
        
--  with Ada.Text_IO; use Ada.Text_IO;

package body My_Integers is

   function Saturate (V : One_To_Ten'Base)
                      return One_To_Ten is
   begin
      --  Put_Line ("SATURATE " & V'Image);

      if V < One_To_Ten'First then
         return One_To_Ten'First;
      elsif V > One_To_Ten'Last then
         return One_To_Ten'Last;
      else
         return V;
      end if;
   end Saturate;

   function Sat_Add (V1, V2 : One_To_Ten'Base)
                     return One_To_Ten is
   begin
      return Saturate (V1 + V2);
   end Sat_Add;

   function Sat_Sub (V1, V2 : One_To_Ten'Base)
                     return One_To_Ten is
   begin
      return Saturate (V1 - V2);
   end Sat_Sub;

end My_Integers;

    
        
with Ada.Text_IO; use Ada.Text_IO;
with My_Integers; use My_Integers;

procedure Show_Base is

   type Display_Saturate_Op is (Add, Sub);

   procedure Display_Saturate
     (V1, V2 : One_To_Ten;
      Op     : Display_Saturate_Op)
   is
      Res : One_To_Ten;
   begin
      case Op is
      when Add =>
         Res := Sat_Add (V1, V2);
      when Sub =>
         Res := Sat_Sub (V1, V2);
      end case;
      Put_Line ("SATURATE " & Op'Image
                & " (" & V1'Image
                & ", " & V2'Image
                & ") = " & Res'Image);
   end Display_Saturate;

begin
   Display_Saturate (1,  1, Add);
   Display_Saturate (10, 8, Add);
   Display_Saturate (1,  8, Sub);
end Show_Base;

In this example, we're using the Base attribute to declare the parameters of the Sat_Add, Sat_Sub and Saturate functions. Note that the parameters of the Display_Saturate procedure are of One_To_Ten type, while the parameters of the Sat_Add, Sat_Sub and Saturate functions are of the (unconstrained) base subtype (One_To_Ten'Base). In those functions, we perform operations using the parameters of unconstrained subtype and adjust the result — in the Saturate function — before returning it as a constrained value of One_To_Ten subtype.

The code in the body of the My_Integers package contains lines that were commented out — to be more precise, a call to Put_Line call in the Saturate function. If you uncomment them, you'll see the value of the input parameter V (of One_To_Ten'Base type) in the runtime output of the program before it's adapted to fit the constraints of the One_To_Ten subtype.

Enumerations

We've introduced enumerations back in the Introduction to Ada course. In this section, we'll discuss a few useful features of enumerations, such as enumeration renaming, enumeration overloading and representation clauses.

In the Ada Reference Manual

Enumerations as functions

If you have used programming language such as C in the past, you're familiar with the concept of enumerations being constants with integer values. In Ada, however, enumerations are not integers. In fact, they're actually parameterless functions! Let's consider this example:

    
    
    
        
package Days is

   type Day is (Mon, Tue, Wed,
                Thu, Fri,
                Sat, Sun);

   --  Essentially, we're declaring
   --  these functions:
   --
   --  function Mon return Day;
   --  function Tue return Day;
   --  function Wed return Day;
   --  function Thu return Day;
   --  function Fri return Day;
   --  function Sat return Day;
   --  function Sun return Day;

end Days;

In the package Days, we're declaring the enumeration type Day. When we do this, we're essentially declaring seven parameterless functions, one for each enumeration. For example, the Mon enumeration corresponds to function Mon return Day. You can see all seven function declarations in the comments of the example above.

Note that this has no direct relation to how an Ada compiler generates machine code for enumeration. Even though enumerations are parameterless functions, a typical Ada compiler doesn't generate function calls for code that deals with enumerations.

Enumeration renaming

The idea that enumerations are parameterless functions can be used when we want to rename enumerations. For example, we could rename the enumerations of the Day type like this:

    
    
    
        
package Enumeration_Example is

   type Day is (Mon, Tue, Wed,
                Thu, Fri,
                Sat, Sun);

   function Monday    return Day renames Mon;
   function Tuesday   return Day renames Tue;
   function Wednesday return Day renames Wed;
   function Thursday  return Day renames Thu;
   function Friday    return Day renames Fri;
   function Saturday  return Day renames Sat;
   function Sunday    return Day renames Sun;

end Enumeration_Example;

Now, we can use both Monday or Mon to refer to Monday of the Day type:

    
    
    
        
with Ada.Text_IO;         use Ada.Text_IO;
with Enumeration_Example; use Enumeration_Example;

procedure Show_Renaming is
   D1 : constant Day := Mon;
   D2 : constant Day := Monday;
begin
   if D1 = D2 then
      Put_Line ("D1 = D2");
      Put_Line (Day'Image (D1)
                & " =  "
                & Day'Image (D2));
   end if;
end Show_Renaming;

When running this application, we can confirm that D1 is equal to D2. Also, even though we've assigned Monday to D2 (instead of Mon), the application displays Mon = Mon, since Monday is just another name to refer to the actual enumeration (Mon).

Hint

If you just want to have a single (renamed) enumeration visible in your application — and make the original enumeration invisible —, you can use a separate package. For example:

    
    
    
        
package Enumeration_Example is

   type Day is (Mon, Tue, Wed,
                Thu, Fri,
                Sat, Sun);

end Enumeration_Example;

    
        
with Enumeration_Example;

package Enumeration_Renaming is

   subtype Day is Enumeration_Example.Day;

   function Monday    return Day renames
     Enumeration_Example.Mon;
   function Tuesday   return Day renames
     Enumeration_Example.Tue;
   function Wednesday return Day renames
     Enumeration_Example.Wed;
   function Thursday  return Day renames
     Enumeration_Example.Thu;
   function Friday    return Day renames
     Enumeration_Example.Fri;
   function Saturday  return Day renames
     Enumeration_Example.Sat;
   function Sunday    return Day renames
     Enumeration_Example.Sun;

end Enumeration_Renaming;

    
        
with Ada.Text_IO; use Ada.Text_IO;

with Enumeration_Renaming;
use  Enumeration_Renaming;

procedure Show_Renaming is
   D1 : constant Day := Monday;
begin
   Put_Line (Day'Image (D1));
end Show_Renaming;

Note that the call to Put_Line still display Mon instead of Monday.

Enumeration overloading

Enumerations can be overloaded. In simple terms, this means that the same name can be used to declare an enumeration of different types. A typical example is the declaration of colors:

    
    
    
        
package Colors is

   type Color is
     (Salmon,
      Firebrick,
      Red,
      Darkred,
      Lime,
      Forestgreen,
      Green,
      Darkgreen,
      Blue,
      Mediumblue,
      Darkblue);

   type Primary_Color is
     (Red,
      Green,
      Blue);

end Colors;

Note that we have Red as an enumeration of type Color and of type Primary_Color. The same applies to Green and Blue. Because Ada is a strongly-typed language, in most cases, the enumeration that we're referring to is clear from the context. For example:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;
with Colors;      use Colors;

procedure Red_Colors is
   C1 : constant Color         := Red;
   --  Using Red from Color

   C2 : constant Primary_Color := Red;
   --  Using Red from Primary_Color
begin
   if C1 = Red then
      Put_Line ("C1 = Red");
   end if;
   if C2 = Red then
      Put_Line ("C2 = Red");
   end if;
end Red_Colors;

When assigning Red to C1 and C2, it is clear that, in the first case, we're referring to Red of Color type, while in the second case, we're referring to Red of the Primary_Color type. The same logic applies to comparisons such as the one in if C1 = Red: because the type of C1 is defined (Color), it's clear that the Red enumeration is the one of Color type.

Enumeration subtypes

Note that enumeration overloading is not the same as enumeration subtypes. For example, we could define the following subtype:

    
    
    
        
package Colors.Shades is

   subtype Blue_Shades is
     Colors range Blue .. Darkblue;

end Colors.Shades;

In this case, Blue of Blue_Shades and Blue of Colors are the same enumeration.

Enumeration ambiguities

A situation where enumeration overloading might lead to ambiguities is when we use them in ranges. For example:

    
    
    
        
package Colors is

   type Color is
     (Salmon,
      Firebrick,
      Red,
      Darkred,
      Lime,
      Forestgreen,
      Green,
      Darkgreen,
      Blue,
      Mediumblue,
      Darkblue);

   type Primary_Color is
     (Red,
      Green,
      Blue);

end Colors;

    
        
with Ada.Text_IO; use Ada.Text_IO;
with Colors;      use Colors;

procedure Color_Loop is
begin
   for C in Red .. Blue loop
   --       ^^^^^^^^^^^
   --  ERROR: range is ambiguous!
      Put_Line (Color'Image (C));
   end loop;
end Color_Loop;

Here, it's not clear whether the range in the loop is of Color type or of Primary_Color type. Therefore, we get a compilation error for this code example. The next line in the code example — the one with the call to Put_Line — gives us a hint about the developer's intention to refer to the Color type. In this case, we can use qualification — for example, Color'(Red) — to resolve the ambiguity:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;
with Colors;      use Colors;

procedure Color_Loop is
begin
   for C in Color'(Red) .. Color'(Blue) loop
      Put_Line (Color'Image (C));
   end loop;
end Color_Loop;

Note that, in the case of ranges, we can also rewrite the loop by using a range declaration:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;
with Colors;      use Colors;

procedure Color_Loop is
begin
   for C in Color range Red .. Blue loop
      Put_Line (Color'Image (C));
   end loop;
end Color_Loop;

Alternatively, Color range Red .. Blue could be used in a subtype declaration, so we could rewrite the example above using a subtype (such as Red_To_Blue) in the loop:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;
with Colors;      use Colors;

procedure Color_Loop is
   subtype Red_To_Blue is Color range Red .. Blue;
begin
   for C in Red_To_Blue loop
      Put_Line (Color'Image (C));
   end loop;
end Color_Loop;

Position and Internal Code

As we've said above, a typical Ada compiler doesn't generate function calls for code that deals with enumerations. On the contrary, each enumeration has values associated with it, and the compiler uses those values instead.

Each enumeration has:

  • a position value, which is a natural value indicating the position of the enumeration in the enumeration type; and

  • an internal code, which, by default, in most cases, is the same as the position value.

Also, by default, the value of the first position is zero, the value of the second position is one, and so on. We can see this by listing each enumeration of the Day type and displaying the value of the corresponding position:

    
    
    
        
package Days is

   type Day is (Mon, Tue, Wed,
                Thu, Fri,
                Sat, Sun);

end Days;

    
        
with Ada.Text_IO; use Ada.Text_IO;
with Days;        use Days;

procedure Show_Days is
begin
   for D in Day loop
      Put_Line (Day'Image (D)
                & " position      = "
                & Integer'Image (Day'Pos (D)));
      Put_Line (Day'Image (D)
                & " internal code = "
                & Integer'Image
                    (Day'Enum_Rep (D)));
   end loop;
end Show_Days;

Note that this application also displays the internal code, which, in this case, is equivalent to the position value for all enumerations.

We may, however, change the internal code of an enumeration using a representation clause. We discuss this topic in another section.

Definite and Indefinite Subtypes

Indefinite types were mentioned back in the Introduction to Ada course. In this section, we'll recapitulate and extend on both definite and indefinite types.

Definite types are the basic kind of types we commonly use when programming applications. For example, we can only declare variables of definite types; otherwise, we get a compilation error. Interestingly, however, to be able to explain what definite types are, we need to first discuss indefinite types.

Indefinite types include:

  • unconstrained arrays;

  • record types with unconstrained discriminants without defaults.

Let's see some examples of indefinite types:

    
    
    
        
package Unconstrained_Types is

   type Integer_Array is
     array (Positive range <>) of Integer;

   type Simple_Record (Extended : Boolean) is
   record
      V : Integer;
      case Extended is
         when False =>
            null;
         when True  =>
            V_Float : Float;
      end case;
   end record;

end Unconstrained_Types;

In this example, both Integer_Array and Simple_Record are indefinite types.

Important

Note that we cannot use indefinite subtypes as discriminants. For example, the following code won't compile:

    
    
    
        
package Unconstrained_Types is

   type Integer_Array is
     array (Positive range <>) of Integer;

   type Simple_Record (Arr : Integer_Array) is
   record
      L : Natural := Arr'Length;
   end record;

end Unconstrained_Types;

Integer_Array is a correct type declaration — although the type itself is indefinite after the declaration. However, we cannot use it as the discriminant in the declaration of Simple_Record. We could, however, have a correct declaration by using discriminants as access values:

    
    
    
        
package Unconstrained_Types is

   type Integer_Array is
     array (Positive range <>) of Integer;

   type Integer_Array_Access is
     access Integer_Array;

   type Simple_Record
     (Arr : Integer_Array_Access) is
   record
      L : Natural := Arr'Length;
   end record;

end Unconstrained_Types;

By adding the Integer_Array_Access type and using it in Simple_Record's type declaration, we can indirectly use an indefinite type in the declaration of another indefinite type. We discuss this topic later in another chapter.

As we've just mentioned, we cannot declare variable of indefinite types:

    
    
    
        
with Unconstrained_Types; use Unconstrained_Types;

procedure Using_Unconstrained_Type is

   A : Integer_Array;

   R : Simple_Record;

begin
   null;
end Using_Unconstrained_Type;

As we can see when we try to build this example, the compiler complains about the declaration of A and R because we're trying to use indefinite types to declare variables. The main reason we cannot use indefinite types here is that the compiler needs to know at this point how much memory it should allocate. Therefore, we need to provide the information that is missing. In other words, we need to change the declaration so the type becomes definite. We can do this by either declaring a definite type or providing constraints in the variable declaration. For example:

    
    
    
        
with Unconstrained_Types; use Unconstrained_Types;

procedure Using_Unconstrained_Type is

   subtype Integer_Array_5 is
     Integer_Array (1 .. 5);

   A1 : Integer_Array_5;
   A2 : Integer_Array (1 .. 5);

   subtype Simple_Record_Ext is
     Simple_Record (Extended => True);

   R1 : Simple_Record_Ext;
   R2 : Simple_Record (Extended => True);

begin
   null;
end Using_Unconstrained_Type;

In this example, we declare the Integer_Array_5 subtype, which is definite because we're constraining it to a range from 1 to 5, thereby defining the information that was missing in the indefinite type Integer_Array. Because we now have a definite type, we can use it to declare the A1 variable. Similarly, we can use the indefinite type Integer_Array directly in the declaration of A2 by specifying the previously unknown range.

Similarly, in this example, we declare the Simple_Record_Ext subtype, which is definite because we're initializing the record discriminant Extended. We can therefore use it in the declaration of the R1 variable. Alternatively, we can simply use the indefinite type Simple_Record and specify the information required for the discriminants. This is what we do in the declaration of the R2 variable.

Although we cannot use indefinite types directly in variable declarations, they're very useful to generalize algorithms. For example, we can use them as parameters of a subprogram:

    
    
    
        
with Unconstrained_Types; use Unconstrained_Types;

procedure Show_Integer_Array (A : Integer_Array);

    
        
with Ada.Text_IO; use Ada.Text_IO;

procedure Show_Integer_Array (A : Integer_Array)
is
begin
   for I in A'Range loop
      Put_Line (Positive'Image (I)
                & ": "
                & Integer'Image (A (I)));
   end loop;
   Put_Line ("--------");
end Show_Integer_Array;

    
        
with Unconstrained_Types; use Unconstrained_Types;
with Show_Integer_Array;

procedure Using_Unconstrained_Type is
   A_5  : constant Integer_Array (1 .. 5)  :=
            (1, 2, 3, 4, 5);
   A_10 : constant Integer_Array (1 .. 10) :=
            (1, 2, 3, 4, 5, others => 99);
begin
   Show_Integer_Array (A_5);
   Show_Integer_Array (A_10);
end Using_Unconstrained_Type;

In this particular example, the compiler doesn't know a priori which range is used for the A parameter of Show_Integer_Array. It could be a range from 1 to 5 as used for variable A_5 of the Using_Unconstrained_Type procedure, or it could be a range from 1 to 10 as used for variable A_10, or it could be anything else. Although the parameter A of Show_Integer_Array is unconstrained, both calls to Show_Integer_Array — in Using_Unconstrained_Type procedure — use constrained objects.

Note that we could call the Show_Integer_Array procedure above with another unconstrained parameter. For example:

    
    
    
        
with Unconstrained_Types; use Unconstrained_Types;

procedure Show_Integer_Array_Header
  (AA : Integer_Array;
   HH : String);

    
        
with Ada.Text_IO;         use Ada.Text_IO;
with Show_Integer_Array;

procedure Show_Integer_Array_Header
  (AA : Integer_Array;
   HH : String)
is
begin
   Put_Line (HH);
   Show_Integer_Array (AA);
end Show_Integer_Array_Header;

    
        
with Unconstrained_Types; use Unconstrained_Types;

with Show_Integer_Array_Header;

procedure Using_Unconstrained_Type is
   A_5  : constant Integer_Array (1 .. 5)  :=
            (1, 2, 3, 4, 5);
   A_10 : constant Integer_Array (1 .. 10) :=
            (1, 2, 3, 4, 5, others => 99);
begin
   Show_Integer_Array_Header (A_5,
                              "First example");
   Show_Integer_Array_Header (A_10,
                              "Second example");
end Using_Unconstrained_Type;

In this case, we're calling the Show_Integer_Array procedure with another unconstrained parameter (the AA parameter). However, although we could have a long chain of procedure calls using indefinite types in their parameters, we still use a (definite) object at the beginning of this chain. For example, for the A_5 object, we have this chain:

A_5

    ==> Show_Integer_Array_Header (AA => A_5,
                                   ...);

        ==> Show_Integer_Array (A => AA);

Therefore, at this specific call to Show_Integer_Array, even though A is declared as a parameter of indefinite type, the actual argument is of definite type because A_5 is constrained — and, thus, of definite type.

Note that we can declare variables based on parameters of indefinite type. For example:

    
    
    
        
with Unconstrained_Types; use Unconstrained_Types;

procedure Show_Integer_Array_Plus
  (A : Integer_Array;
   V : Integer);

    
        
with Show_Integer_Array;

procedure Show_Integer_Array_Plus
  (A : Integer_Array;
   V : Integer)
is
   A_Plus : Integer_Array (A'Range);
begin
   for I in A_Plus'Range loop
      A_Plus (I) := A (I) + V;
   end loop;
   Show_Integer_Array (A_Plus);
end Show_Integer_Array_Plus;

    
        
with Unconstrained_Types; use Unconstrained_Types;

with Show_Integer_Array_Plus;

procedure Using_Unconstrained_Type is
   A_5 : constant Integer_Array (1 .. 5) :=
           (1, 2, 3, 4, 5);
begin
   Show_Integer_Array_Plus (A_5, 5);
end Using_Unconstrained_Type;

In the Show_Integer_Array_Plus procedure, we're declaring A_Plus based on the range of A, which is itself of indefinite type. However, since the object passed as an argument to Show_Integer_Array_Plus must have a constraint, A_Plus will also be constrained. For example, in the call to Show_Integer_Array_Plus using A_5 as an argument, the declaration of A_Plus becomes A_Plus : Integer_Array (1 .. 5);. Therefore, it becomes clear that the compiler needs to allocate five elements for A_Plus.

We'll see later how definite and indefinite types apply to formal parameters.

In the Ada Reference Manual

Constrained Attribute

We can use the Constrained attribute to verify whether an object of discriminated type is constrained or not. Let's start our discussion by reusing the Simple_Record type from previous examples. In this version of the Unconstrained_Types package, we're adding a Reset procedure for the discriminated record type:

    
    
    
        
package Unconstrained_Types is

   type Simple_Record
     (Extended : Boolean := False) is
   record
      V : Integer;
      case Extended is
         when False =>
            null;
         when True  =>
            V_Float : Float;
      end case;
   end record;

   procedure Reset (R : in out Simple_Record);

end Unconstrained_Types;

    
        
with Ada.Text_IO; use Ada.Text_IO;

package body Unconstrained_Types is

   procedure Reset (R : in out Simple_Record) is
      Zero_Not_Extended : constant
        Simple_Record := (Extended => False,
                          V        => 0);

      Zero_Extended : constant
        Simple_Record := (Extended => True,
                          V        => 0,
                          V_Float  => 0.0);
   begin
      Put_Line ("---- Reset: R'Constrained => "
                & R'Constrained'Image);

      if not R'Constrained then
         R := Zero_Extended;
      else
         if R.Extended then
            R := Zero_Extended;
         else
            R := Zero_Not_Extended;
         end if;
      end if;
   end Reset;

end Unconstrained_Types;

As the name indicates, the Reset procedure initializes all record components with zero. Note that we use the Constrained attribute to verify whether objects are constrained before assigning to them. For objects that are not constrained, we can simply assign another object to it — as we do with the R := Zero_Extended statement. When an object is constrained, however, the discriminants must match. If we assign an object to R, the discriminant of that object must match the discriminant of R. This is the kind of verification that we do in the else part of that procedure: we check the state of the Extended discriminant before assigning an object to the R parameter.

The Using_Constrained_Attribute procedure below declares two objects of Simple_Record type: R1 and R2. Because the Simple_Record type has a default value for its discriminant, we can declare objects of this type without specifying a value for the discriminant. This is exactly what we do in the declaration of R1. Here, we don't specify any constraints, so that it takes the default value (Extended => False). In the declaration of R2, however, we explicitly set Extended to False:

    
    
    
        
with Ada.Text_IO;         use Ada.Text_IO;

with Unconstrained_Types; use Unconstrained_Types;

procedure Using_Constrained_Attribute is
   R1 : Simple_Record;
   R2 : Simple_Record (Extended => False);

   procedure Show_Rs is
   begin
      Put_Line ("R1'Constrained => "
                & R1'Constrained'Image);
      Put_Line ("R1.Extended => "
                & R1.Extended'Image);
      Put_Line ("--");
      Put_Line ("R2'Constrained => "
                & R2'Constrained'Image);
      Put_Line ("R2.Extended => "
                & R2.Extended'Image);
      Put_Line ("----------------");
   end Show_Rs;
begin
   Show_Rs;

   Reset (R1);
   Reset (R2);
   Put_Line ("----------------");

   Show_Rs;
end Using_Constrained_Attribute;

When we run this code, the user messages from Show_Rs indicate to us that R1 is not constrained, while R2 is constrained. Because we declare R1 without specifying a value for the Extended discriminant, R1 is not constrained. In the declaration of R2, on the other hand, the explicit value for the Extended discriminant makes this object constrained. Note that, for both R1 and R2, the value of Extended is False in the declarations.

As we were just discussing, the Reset procedure includes checks to avoid mismatches in discriminants. When we don't have those checks, we might get exceptions at runtime. We can force this situation by replacing the implementation of the Reset procedure with the following lines:

--  [...]
begin
   Put_Line ("---- Reset: R'Constrained => "
             & R'Constrained'Image);
   R := Zero_Extended;
end Reset;

Running the code now generates a runtime exception:

raised CONSTRAINT_ERROR : unconstrained_types.adb:12 discriminant check failed

This exception is raised during the call to Reset (R2). As see in the code, R2 is constrained. Also, its Extended discriminant is set to False, which means that it doesn't have the V_Float component. Therefore, R2 is not compatible with the constant Zero_Extended object, so we cannot assign Zero_Extended to R2. Also, because R2 is constrained, its Extended discriminant cannot be modified.

The behavior is different for the call to Reset (R1), which works fine. Here, when we pass R1 as an argument to the Reset procedure, its Extended discriminant is False by default. Thus, R1 is also not compatible with the Zero_Extended object. However, because R1 is not constrained, the assignment modifies R1 (by changing the value of the Extended discriminant). Therefore, with the call to Reset, the Extended discriminant of R1 changes from False to True.

In the Ada Reference Manual

Incomplete types

Incomplete types — as the name suggests — are types that have missing information in their declaration. This is a simple example:

type Incomplete;

Because this type declaration is incomplete, we need to provide the missing information at some later point. Consider the incomplete type R in the following example:

    
    
    
        
package Incomplete_Type_Example is

   type R;
   --  Incomplete type declaration!

   type R is record
      I : Integer;
   end record;
   --  type R is now complete!

end Incomplete_Type_Example;

The first declaration of type R is incomplete. However, in the second declaration of R, we specify that R is a record. By providing this missing information, we're completing the type declaration of R.

It's also possible to declare an incomplete type in the private part of a package specification and its complete form in the package body. Let's rewrite the example above accordingly:

    
    
    
        
package Incomplete_Type_Example is

private

   type R;
   --  Incomplete type declaration!

end Incomplete_Type_Example;

    
        
package body Incomplete_Type_Example is

   type R is record
      I : Integer;
   end record;
   --  type R is now complete!

end Incomplete_Type_Example;

A typical application of incomplete types is to create linked lists using access types based on those incomplete types. This kind of type is called a recursive type. For example:

    
    
    
        
package Linked_List_Example is

   type Integer_List;

   type Next is access Integer_List;

   type Integer_List is record
      I : Integer;
      N : Next;
   end record;

end Linked_List_Example;

Here, the N component of Integer_List is essentially giving us access to the next element of Integer_List type. Because the Next type is both referring to the Integer_List type and being used in the declaration of the Integer_List type, we need to start with an incomplete declaration of the Integer_List type and then complete it after the declaration of Next.

Incomplete types are useful to declare mutually dependent types, as we'll see later on. Also, we can also have formal incomplete types, as we'll discuss later.

In the Ada Reference Manual

Type view

Ada distinguishes between the partial and the full view of a type. The full view is a type declaration that contains all the information needed by the compiler. For example, the following declaration of type R represents the full view of this type:

    
    
    
        
package Full_View is

   --  Full view of the R type:
   type R is record
      I : Integer;
   end record;

end Full_View;

As soon as we start applying encapsulation and information hiding — via the private keyword — to a specific type, we are introducing a partial view and making only that view compile-time visible to clients. Doing so requires us to introduce the private part of the package (unless already present). For example:

    
    
    
        
package Partial_Full_Views is

   --  Partial view of the R type:
   type R is private;

private

   --  Full view of the R type:
   type R is record
      I : Integer;
   end record;

end Partial_Full_Views;

As indicated in the example, the type R is private declaration is the partial view of the R type, while the type R is record [...] declaration in the private part of the package is the full view.

Although the partial view doesn't contain the full type declaration, it contains very important information for the users of the package where it's declared. In fact, the partial view of a private type is all that users actually need to know to effectively use this type, while the full view is only needed by the compiler.

In the previous example, the partial view indicates that R is a private type, which means that, even though users cannot directly access any information stored in this type — for example, read the value of the I component of R —, they can use the R type to declare objects. For example:

    
    
    
        
with Partial_Full_Views; use Partial_Full_Views;

procedure Main is
   --  Partial view of R indicates that
   --  R exists as a private type, so we
   --  can declare objects of this type:
   C : R;
begin
   --  But we cannot directly access any
   --  information declared in the full
   --  view of R:
   --
   --  C.I := 42;
   --
   null;
end Main;

In many cases, the restrictions applied to the partial and full views must match. For example, if we declare a limited type in the full view of a private type, its partial view must also be limited:

    
    
    
        
package Limited_Private_Example is

   --  Partial view must be limited,
   --  since the full view is limited.
   type R is limited private;

private

   type R is limited record
      I : Integer;
   end record;

end Limited_Private_Example;

There are, however, situations where the full view may contain additional requirements that aren't mentioned in the partial view. For example, a type may be declared as non-tagged in the partial view, but, at the same time, be tagged in the full view:

    
    
    
        
package Tagged_Full_View_Example is

   --  Partial view using non-tagged type:
   type R is private;

private

   --  Full view using tagged type:
   type R is tagged record
      I : Integer;
   end record;

end Tagged_Full_View_Example;

In this case, from a user's perspective, the R type is non-tagged, so that users cannot use any object-oriented programming features for this type. In the package body of Tagged_Full_View_Example, however, this type is tagged, so that all object-oriented programming features are available for subprograms of the package body that make use of this type. Again, the partial view of the private type contains the most important information for users that want to declare objects of this type.

Important

Although it's very common to declare private types as record types, this is not the only option. In fact, we could declare any type in the full view — scalars, for example —, so we could declare a "private integer" type:

    
    
    
        
package Private_Integers is

   --  Partial view of private Integer type:
   type Private_Integer is private;

private

   --  Full view of private Integer type:
   type Private_Integer is new Integer;

end Private_Integers;

This code compiles as expected, but isn't very useful. We can improve it by adding operators to it, for example:

    
    
    
        
package Private_Integers is

   --  Partial view of private Integer type:
   type Private_Integer is private;

   function "+" (Left, Right : Private_Integer)
                 return Private_Integer;

private

   --  Full view of private Integer type:
   type Private_Integer is new Integer;

end Private_Integers;

    
        
package body Private_Integers is

   function "+" (Left, Right : Private_Integer)
                 return Private_Integer
   is
      Res : constant Integer :=
              Integer (Left) + Integer (Right);
      --  Note that we're converting Left
      --  and Right to Integer, which calls
      --  the "+" operator of the Integer
      --  type. Writing "Left + Right" would
      --  have called the "+" operator of
      --  Private_Integer, which leads to
      --  recursive calls, as this is the
      --  operator we're currently in.
   begin
      return Private_Integer (Res);
   end "+";

end Private_Integers;

Now, we can use the + operator as a common integer variable:

    
    
    
        
with Private_Integers; use Private_Integers;

procedure Show_Private_Integers is
   A, B : Private_Integer;
begin
   A := A + B;
end Show_Private_Integers;

In the Ada Reference Manual

Type conversion

An important operation when dealing with objects of different types is type conversion, which we already discussed in the Introduction to Ada course. In fact, we can convert an object Obj_X of an operand type X to a similar, closely related target type Y by simply indicating the target type: Y (Obj_X). In this section, we discuss type conversions for different kinds of types.

Ada distinguishes between two kinds of conversion: value conversion and view conversion. The main difference is the way how the operand (argument) of the conversion is evaluated:

  • in a value conversion, the operand is evaluated as an expression;

  • in a view conversion, the operand is evaluated as a name.

In other words, we cannot use expressions such as 2 * A in a view conversion, but only A. In a value conversion, we could use both forms.

In the Ada Reference Manual

Value conversion

Value conversions are possible for various types. In this section, we see some examples, starting with types derived from scalar types up to array conversions.

Root and derived types

Let's start with the conversion between a scalar type and its derived types. For example, we can convert back-and-forth between the Integer type and the derived Int type:

    
    
    
        
package Custom_Integers is

   type Int is new Integer
     with Dynamic_Predicate => Int /= 0;

   function Double (I : Integer)
                    return Integer is
     (I * 2);

end Custom_Integers;

    
        
with Ada.Text_IO;     use Ada.Text_IO;
with Custom_Integers; use Custom_Integers;

procedure Show_Conversion is
   Int_Var     : Int     := 1;
   Integer_Var : Integer := 2;
begin
   --  Int to Integer conversion
   Integer_Var := Integer (Int_Var);

   Put_Line ("Integer_Var : "
             & Integer_Var'Image);

   --  Int to Integer conversion
   --  as an actual parameter
   Integer_Var := Double (Integer (Int_Var));

   Put_Line ("Integer_Var : "
             & Integer_Var'Image);

   --  Integer to Int conversion
   --  using an expression
   Int_Var     := Int (Integer_Var * 2);

   Put_Line ("Int_Var :     "
             & Int_Var'Image);
end Show_Conversion;

In the Show_Conversion procedure from this example, we first convert from Int to Integer. Then, we do the same conversion while providing the resulting value as an actual parameter for the Double function. Finally, we convert the Integer_Var * 2 expression from Integer to Int.

Note that the converted value must conform to any constraints that the target type might have. In the example above, Int has a predicate that dictates that its value cannot be zero. This (dynamic) predicate is checked at runtime, so an exception is raised if it fails:

    
    
    
        
with Ada.Text_IO;     use Ada.Text_IO;
with Custom_Integers; use Custom_Integers;

procedure Show_Conversion is
   Int_Var     : Int;
   Integer_Var : Integer;
begin
   Integer_Var := 0;
   Int_Var     := Int (Integer_Var);

   Put_Line ("Int_Var : "
             & Int_Var'Image);
end Show_Conversion;

In this case, the conversion from Integer to Int fails because, while zero is a valid integer value, it doesn't obey Int's predicate.

Numeric type conversion

A typical conversion is the one between integer and floating-point values. For example:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;

procedure Show_Conversion is
   F : Float   := 1.0;
   I : Integer := 2;
begin
   I := Integer (F);

   Put_Line ("I : "
             & I'Image);

   I := 4;
   F := Float (I);

   Put_Line ("F :   "
             & F'Image);
end Show_Conversion;

Also, we can convert between fixed-point types and floating-point or integer types:

    
    
    
        
package Fixed_Point_Defs is
   S     : constant := 32;
   Exp   : constant := 15;
   D     : constant := 2.0 ** (-S + Exp + 1);

   type TQ15_31 is delta D
     range -1.0 * 2.0 ** Exp ..
            1.0 * 2.0 ** Exp - D;

   pragma Assert (TQ15_31'Size = S);
end Fixed_Point_Defs;

    
        
with Fixed_Point_Defs; use Fixed_Point_Defs;
with Ada.Text_IO;      use Ada.Text_IO;

procedure Show_Conversion is
   F  : Float;
   FP : TQ15_31;
   I  : Integer;
begin
   FP := TQ15_31 (10.25);
   I  := Integer (FP);

   Put_Line ("FP : "
             & FP'Image);
   Put_Line ("I : "
             & I'Image);

   I  := 128;
   FP := TQ15_31 (I);
   F  := Float (FP);

   Put_Line ("FP : "
             & FP'Image);
   Put_Line ("F :   "
             & F'Image);
end Show_Conversion;

As we can see in the examples above, converting between different numeric types works in all directions. (Of course, rounding is applied when converting from floating-point to integer types, but this is expected.)

Enumeration conversion

We can also convert between an enumeration type and a type derived from it:

    
    
    
        
package Custom_Enumerations is

   type Priority is (Low, Mid, High);

   type Important_Priority is new
     Priority range Mid .. High;

end Custom_Enumerations;

    
        
with Ada.Text_IO;         use Ada.Text_IO;
with Custom_Enumerations; use Custom_Enumerations;

procedure Show_Conversion is
   P  : Priority           := Low;
   IP : Important_Priority := High;
begin
   P := Priority (IP);

   Put_Line ("P:  "
             & P'Image);

   P  := Mid;
   IP := Important_Priority (P);

   Put_Line ("IP: "
             & IP'Image);
end Show_Conversion;

In this example, we have the Priority type and the derived type Important_Priority. As expected, the conversion works fine when the converted value is in the range of the target type. If not, an exception is raised:

    
    
    
        
with Ada.Text_IO;         use Ada.Text_IO;
with Custom_Enumerations; use Custom_Enumerations;

procedure Show_Conversion is
   P  : Priority;
   IP : Important_Priority;
begin
   P  := Low;
   IP := Important_Priority (P);

   Put_Line ("IP: "
             & IP'Image);
end Show_Conversion;

In this example, an exception is raised because Low is not in the Important_Priority type's range.

Array conversion

Similarly, we can convert between array types. For example, if we have the array type Integer_Array and its derived type Derived_Integer_Array, we can convert between those array types:

    
    
    
        
package Custom_Arrays is

   type Integer_Array is
     array (Positive range <>) of Integer;

   type Derived_Integer_Array is new
     Integer_Array;

end Custom_Arrays;

    
        
pragma Ada_2022;

with Ada.Text_IO;   use Ada.Text_IO;
with Custom_Arrays; use Custom_Arrays;

procedure Show_Conversion is
   subtype Common_Range is Positive range 1 .. 3;

   AI : Integer_Array (Common_Range);
   AI_D : Derived_Integer_Array (Common_Range);
begin
   AI_D := [1, 2, 3];
   AI := Integer_Array (AI_D);

   Put_Line ("AI: "
             & AI'Image);

   AI   := [4, 5, 6];
   AI_D := Derived_Integer_Array (AI);

   Put_Line ("AI_D: "
             & AI_D'Image);
end Show_Conversion;

Note that both arrays must have the same number of components in order for the conversion to be successful. (Sliding is fine, though.) In this example, both arrays have the same range: Common_Range.

We can also convert between array types that aren't derived one from the other. As long as the components and the index subtypes are of the same type, the conversion between those types is possible. To be more precise, these are the requirements for the array conversion to be accepted:

  • The component types must be the same type.

  • The index types (or subtypes) must be the same or, at least, convertible.

  • The dimensionality of the arrays must be the same.

  • The bounds must be compatible (but not necessarily equal).

Converting between different array types can be very handy, especially when we're dealing with array types that were not declared in the same package. For example:

    
    
    
        
package Custom_Arrays_1 is

   type Integer_Array_1 is
     array (Positive range <>) of Integer;

   type Float_Array_1 is
     array (Positive range <>) of Float;

end Custom_Arrays_1;

    
        
package Custom_Arrays_2 is

   type Integer_Array_2 is
     array (Positive range <>) of Integer;

   type Float_Array_2 is
     array (Positive range <>) of Float;

end Custom_Arrays_2;

    
        
pragma Ada_2022;

with Ada.Text_IO;     use Ada.Text_IO;
with Custom_Arrays_1; use Custom_Arrays_1;
with Custom_Arrays_2; use Custom_Arrays_2;

procedure Show_Conversion is
   subtype Common_Range is Positive range 1 .. 3;

   AI_1 : Integer_Array_1 (Common_Range);
   AI_2 : Integer_Array_2 (Common_Range);
   AF_1 : Float_Array_1 (Common_Range);
   AF_2 : Float_Array_2 (Common_Range);
begin
   AI_2 := [1, 2, 3];
   AI_1 := Integer_Array_1 (AI_2);

   Put_Line ("AI_1: "
             & AI_1'Image);

   AI_1 := [4, 5, 6];
   AI_2 := Integer_Array_2 (AI_1);

   Put_Line ("AI_2: "
             & AI_2'Image);

   --  ERROR: Cannot convert arrays whose
   --         components have different types:
   --
   --  AF_1 := Float_Array_1 (AI_1);
   --
   --  Instead, use array aggregate where each
   --  component is converted from integer to
   --  float:
   --
   AF_1 := [for I in AF_1'Range =>
              Float (AI_1 (I))];

   Put_Line ("AF_1: "
             & AF_1'Image);

   AF_2 := Float_Array_2 (AF_1);

   Put_Line ("AF_2: "
             & AF_2'Image);
end Show_Conversion;

As we can see in this example, the fact that Integer_Array_1 and Integer_Array_2 have the same component type (Integer) allows us to convert between them. The same applies to the Float_Array_1 and Float_Array_2 types.

A conversion is not possible when the component types don't match. Even though we can convert between integer and floating-point types, we cannot convert an array of integers to an array of floating-point directly. Therefore, we cannot write a statement such as AF_1 := Float_Array_1 (AI_1);.

However, when the components don't match, we can of course implement the array conversion by converting the individual components. For the example above, we used an iterated component association in an array aggregate: [for I in AF_1'Range => Float (AI_1 (I))];. (We discuss this topic later in another chapter.)

We may also encounter array types originating from the instantiation of generic packages. In this case as well, we can use array conversions. Consider the following generic package:

    
    
    
        
generic
   type T is private;
package Custom_Arrays is
   type T_Array is
     array (Positive range <>) of T;
end Custom_Arrays;

We could instantiate this generic package and reuse parts of the previous code example:

    
    
    
        
pragma Ada_2022;

with Ada.Text_IO;   use Ada.Text_IO;
with Custom_Arrays;

procedure Show_Conversion is
   package CA_Int_1 is
     new Custom_Arrays (T => Integer);
   package CA_Int_2 is
     new Custom_Arrays (T => Integer);

   subtype Common_Range is Positive range 1 .. 3;

   AI_1 : CA_Int_1.T_Array (Common_Range);
   AI_2 : CA_Int_2.T_Array (Common_Range);
begin
   AI_2 := [1, 2, 3];
   AI_1 := CA_Int_1.T_Array (AI_2);

   Put_Line ("AI_1: "
             & AI_1'Image);

   AI_1 := [4, 5, 6];
   AI_2 := CA_Int_2.T_Array (AI_1);

   Put_Line ("AI_2: "
             & AI_2'Image);
end Show_Conversion;

As we can see in this example, each of the instantiated CA_Int_1 and CA_Int_2 packages has a T_Array type. Even though these T_Array types have the same name, they're actually completely unrelated types. However, we can still convert between them in the same way as we did in the previous code examples.

View conversion

As mentioned before, view conversions just allow names to be converted. Thus, we cannot use expressions in this case.

Note that a view conversion never changes the value during the conversion. We could say that a view conversion is simply making us view an object from a different angle. The object itself is still the same for both the original and the target types.

For example, consider this package:

    
    
    
        
package Some_Tagged_Types is

   type T is tagged record
      A : Integer;
   end record;

   type T_Derived is new T with record
      B : Float;
   end record;

   Obj : T_Derived;

end Some_Tagged_Types;

Here, Obj is an object of type T_Derived. When we view this object, we notice that it has two components: A and B. However, we could view this object as being of type T. From that perspective, this object only has one component: A. (Note that changing the perspective doesn't change the object itself.) Therefore, a view conversion from T_Derived to T just makes us view the object Obj from a different angle.

In this sense, a view conversion changes the view of a given object to the target type's view, both in terms of components that exist and operations that are available. It doesn't really change anything at all in the value itself.

There are basically two kinds of view conversions: the ones using tagged types and the ones using untagged types. We discuss these kinds of conversion in this section.

View conversion of tagged types

A conversion between tagged types is a view conversion. Let's consider a typical code example that declares one, two and three-dimensional points:

    
    
    
        
package Points is

   type Point_1D is tagged record
      X : Float;
   end record;

   procedure Display (P : Point_1D);

   type Point_2D is new Point_1D with record
      Y : Float;
   end record;

   procedure Display (P : Point_2D);

   type Point_3D is new Point_2D with record
      Z : Float;
   end record;

   procedure Display (P : Point_3D);

end Points;

    
        
with Ada.Text_IO; use Ada.Text_IO;

package body Points is

   procedure Display (P : Point_1D) is
   begin
      Put_Line ("(X => " & P.X'Image & ")");
   end Display;

   procedure Display (P : Point_2D) is
   begin
      Put_Line ("(X => " & P.X'Image
                & ", Y => " & P.Y'Image & ")");
   end Display;

   procedure Display (P : Point_3D) is
   begin
      Put_Line ("(X => " & P.X'Image
                & ", Y => " & P.Y'Image
                & ", Z => " & P.Z'Image & ")");
   end Display;

end Points;

We can use the types from the Points package and convert between each other:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;
with Points;      use Points;

procedure Show_Conversion is
   P_1D : Point_1D;
   P_3D : Point_3D;
begin
   P_3D := (X => 0.1, Y => 0.5, Z => 0.3);
   P_1D := Point_1D (P_3D);

   Put ("P_3D : ");
   Display (P_3D);

   Put ("P_1D : ");
   Display (P_1D);
end Show_Conversion;

In this example, as expected, we're able to convert from the Point_3D type (which has three components) to the Point_1D type, which has only one component.

View conversion of untagged types

For untagged types, a view conversion is the one that happens when we have an object of an untagged type as an actual parameter for a formal in out or out parameter.

Let's see a code example. Consider the following simple procedure:

    
    
    
        
procedure Double (X : in out Float);

    
        
procedure Double (X : in out Float) is
begin
   X := X * 2.0;
end Double;

The Double procedure has an in out parameter of Float type. We can call this procedure using an integer variable I as the actual parameter. For example:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;
with Double;

procedure Show_Conversion is
   I : Integer;
begin
   I := 2;
   Put_Line ("I : "
             & I'Image);

   --  Calling Double with
   --  Integer parameter:
   Double (Float (I));
   Put_Line ("I : "
             & I'Image);
end Show_Conversion;

In this case, the Float (I) conversion in the call to Double creates a temporary floating-point variable. This is the same as if we had written the following code:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;
with Double;

procedure Show_Conversion is
   I : Integer;
begin
   I := 2;
   Put_Line ("I : "
             & I'Image);

   declare
      F : Float := Float (I);
   begin
      Double (F);
      I := Integer (F);
   end;
   Put_Line ("I : "
             & I'Image);
end Show_Conversion;

In this sense, the view conversion that happens in Double (Float (I)) can be considered syntactic sugar, as it allows us to elegantly write two conversions in a single statement.

Implicit conversions

Implicit conversions are only possible when we have a type T and a subtype S related to the T type. For example:

    
    
    
        
package Custom_Integers is

   type Int is new Integer
     with Dynamic_Predicate => Int /= 0;

   subtype Sub_Int_1 is Integer
     with Dynamic_Predicate => Sub_Int_1 /= 0;

   subtype Sub_Int_2 is Sub_Int_1
     with Dynamic_Predicate => Sub_Int_2 /= 1;

end Custom_Integers;

    
        
with Ada.Text_IO;     use Ada.Text_IO;
with Custom_Integers; use Custom_Integers;

procedure Show_Conversion is
   Int_Var       : Int;
   Sub_Int_1_Var : Sub_Int_1;
   Sub_Int_2_Var : Sub_Int_2;
   Integer_Var   : Integer;
begin
   Integer_Var := 5;
   Int_Var     := Int (Integer_Var);

   Put_Line ("Int_Var :       "
             & Int_Var'Image);

   --  Implicit conversions:
   --  no explicit conversion required!
   Sub_Int_1_Var := Integer_Var;
   Sub_Int_2_Var := Integer_Var;

   Put_Line ("Sub_Int_1_Var : "
             & Sub_Int_1_Var'Image);
   Put_Line ("Sub_Int_2_Var : "
             & Sub_Int_2_Var'Image);
end Show_Conversion;

In this example, we declare the Int type and the Sub_Int_1 and Sub_Int_2 subtypes:

  • the Int type is derived from the Integer type,

  • Sub_Int_1 is a subtype of the Integer type, and

  • Sub_Int_2 is a subtype of the Sub_Int_1 subtype.

We need an explicit conversion when converting between the Integer and Int types. However, as the conversion is implicit for subtypes, we can simply write Sub_Int_1_Var := Integer_Var;. (Of course, writing the explicit conversion Sub_Int_1 (Integer_Var) in the assignment is possible as well.) Also, the same applies to the Sub_Int_2 subtype: we can write an implicit conversion in the Sub_Int_2_Var := Integer_Var; statement.

Conversion of other types

For other kinds of types, such as records, a direct conversion as we've seen so far isn't possible. In this case, we have to write a conversion function ourselves. A common convention in Ada is to name this function To_Typename. For example, if we want to convert from any type to Integer or Float, we implement the To_Integer and To_Float functions, respectively. (Obviously, because Ada supports subprogram overloading, we can have multiple To_Typename functions for different operand types.)

Let's see a code example:

    
    
    
        
package Custom_Rec is

   type Rec is record
      X : Integer;
   end record;

   function To_Integer (R : Rec)
                        return Integer is
     (R.X);

end Custom_Rec;

    
        
with Ada.Text_IO; use Ada.Text_IO;
with Custom_Rec;  use Custom_Rec;

procedure Show_Conversion is
   R : Rec;
   I : Integer;
begin
   R := (X => 2);
   I := To_Integer (R);

   Put_Line ("I : " & I'Image);
end Show_Conversion;

In this example, we have the To_Integer function that converts from the Rec type to the Integer type.

In other languages

In C++, you can define conversion operators to cast between objects of different classes. Also, you can overload the = operator. Consider this example:

#include <iostream>

class T1 {
public:
    T1 (float x) :
      x(x) {}

    // If class T3 is declared before class
    // T1, we can overload the "=" operator.
    //
    // void operator=(T3 v) {
    //     x = static_cast<float>(v);
    // }

    void display();
private:
   float x;
};

class T3 {
public:
    T3 (float x, float y, float z) :
      x(x), y(y), z(z) {}

    // implicit conversion
    operator float() const {
        return (x + y + z) / 3.0;
    }

    // implicit conversion
    //
    // operator T1() const {
    //     return T1((x + y + z) / 3.0);
    // }

    // explicit conversion (C++11)
    explicit operator T1() const {
        return T1(float(*this));
    }

    void display();

private:
    float x, y, z;
};

void T1::display()
{
    std::cout << "(x => " << x
              << ")" << std::endl;
}

void T3::display()
{
    std::cout << "(x => " << x
              << "y => "  << y
              << "z => "  << z
              << ")" << std::endl;
}

int main ()
{
    const T3 t_3 (0.5, 0.4, 0.6);
    T1 t_1 (0.0);
    float f;

    // Implicit conversion
    f = t_3;

    std::cout << "f : " << f
              << std::endl;

    // Explicit conversion
    f = static_cast<float>(t_3);

    // f = (float)t_3;

    std::cout << "f : " << f
              << std::endl;

    // Explicit conversion
    t_1 = static_cast<T1>(t_3);

    // t_1 = (T1)t_3;

    std::cout << "t_1 : ";
    t_1.display();
    std::cout << std::endl;
}

Here, we're using operator float() and operator T1() to cast from an object of class T3 to a floating-point value and an object of class T1, respectively. (If we switch the order and declare the T3 class before the T1 class, we could overload the = operator, as you can see in the commented-out lines.)

In Ada, this kind of conversions isn't available. Instead, we have to implement conversion functions such as the To_Integer function from the previous code example. This is the corresponding implementation:

    
    
    
        
package Custom_Defs is

   type T1 is private;

   function Init (X : Float)
                  return T1;

   procedure Display (Obj : T1);

   type T3 is private;

   function Init (X, Y, Z : Float)
                  return T3;

   function To_Float (Obj : T3)
                      return Float;

   function To_T1 (Obj : T3)
                   return T1;

   procedure Display (Obj : T3);

private
   type T1 is record
      X : Float;
   end record;

   function Init (X : Float)
                  return T1 is
     (X => X);

   type T3 is record
      X, Y, Z : Float;
   end record;

   function Init (X, Y, Z : Float)
                  return T3 is
     (X => X, Y => Y, Z => Z);

end Custom_Defs;

    
        
with Ada.Text_IO; use Ada.Text_IO;

package body Custom_Defs is

   procedure Display (Obj : T1) is
   begin
      Put_Line ("(X => "
                & Obj.X'Image & ")");
   end Display;

   function To_Float (Obj : T3)
                      return Float is
     ((Obj.X + Obj.Y + Obj.Z) / 3.0);

   function To_T1 (Obj : T3)
                   return T1 is
     (Init (To_Float (Obj)));

   procedure Display (Obj : T3) is
   begin
      Put_Line ("(X => "    & Obj.X'Image
                & ", Y => " & Obj.Y'Image
                & ", Z => " & Obj.Z'Image
                & ")");
   end Display;

end Custom_Defs;

    
        
with Ada.Text_IO; use Ada.Text_IO;
with Custom_Defs; use Custom_Defs;

procedure Show_Conversion is
   T_3 : constant T3 := Init (0.5, 0.4, 0.6);
   T_1 :          T1 := Init (0.0);
   F   : Float;
begin
   --  Explicit conversion from
   --  T3 to Float type
   F := To_Float (T_3);

   Put_Line ("F : " & F'Image);

   --  Explicit conversion from
   --  T3 to T1 type
   T_1 := To_T1 (T_3);

   Put ("T_1 : ");
   Display (T_1);
end Show_Conversion;

In this example, we translate the casting operators from the C++ version by implementing the To_Float and To_T1 functions. (In addition to that, we replace the C++ constructors by Init functions.)

Qualified Expressions

We already saw qualified expressions in the Introduction to Ada course. As mentioned there, a qualified expression specifies the exact type or subtype that the target expression will be resolved to, and it can be either any expression in parentheses, or an aggregate:

    
    
    
        
package Simple_Integers is

   type Int is new Integer;

   subtype Int_Not_Zero is Int
     with Dynamic_Predicate => Int_Not_Zero /= 0;

end Simple_Integers;

    
        
with Simple_Integers; use Simple_Integers;

procedure Show_Qualified_Expressions is
   I : Int;
begin
   --  Using qualified expression Int'(N)
   I := Int'(0);
end Show_Qualified_Expressions;

Here, the qualified expression Int'(0) indicates that the value zero is of Int type.

In the Ada Reference Manual

Verifying subtypes

Note

This feature was introduced in Ada 2022.

We can use qualified expressions to verify a subtype's predicate:

    
    
    
        
with Simple_Integers; use Simple_Integers;

procedure Show_Qualified_Expressions is
   I : Int;
begin
   I := Int_Not_Zero'(0);
end Show_Qualified_Expressions;

Here, the qualified expression Int_Not_Zero'(0) checks the dynamic predicate of the subtype. (This predicate check fails at runtime.)

Default initial values

In the Introduction to Ada course, we've seen that record components can have default values. For example:

    
    
    
        
package Defaults is

   type R is record
     X : Positive := 1;
     Y : Positive := 10;
   end record;

end Defaults;

In this section, we'll extend the concept of default values to other kinds of type declarations, such as scalar types and arrays.

To assign a default value for a scalar type declaration — such as an enumeration and a new integer —, we use the Default_Value aspect:

    
    
    
        
package Defaults is

   type E is (E1, E2, E3)
     with Default_Value => E1;

   type T is new Integer
     with Default_Value => -1;

end Defaults;

Note that we cannot specify a default value for a subtype:

    
    
    
        
package Defaults is

   subtype T is Integer
     with Default_Value => -1;
   --  ERROR!!

end Defaults;

For array types, we use the Default_Component_Value aspect:

    
    
    
        
package Defaults is

   type Arr is
     array (Positive range <>) of Integer
       with Default_Component_Value => -1;

end Defaults;

This is a package containing the declarations we've just seen:

    
    
    
        
package Defaults is

   type E is (E1, E2, E3)
     with Default_Value => E1;

   type T is new Integer
     with Default_Value => -1;

   --  We cannot specify default
   --  values for subtypes:
   --
   --  subtype T is Integer
   --    with Default_Value => -1;

   type R is record
     X : Positive := 1;
     Y : Positive := 10;
   end record;

   type Arr is
     array (Positive range <>) of Integer
       with Default_Component_Value => -1;

end Defaults;

In the example below, we declare variables of the types from the Defaults package:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;
with Defaults; use Defaults;

procedure Use_Defaults is
   E1 : E;
   T1 : T;
   R1 : R;
   A1 : Arr (1 .. 5);
begin
   Put_Line ("Enumeration:  "
             & E'Image (E1));
   Put_Line ("Integer type: "
             & T'Image (T1));
   Put_Line ("Record type:  "
             & Positive'Image (R1.X)
             & ", "
             & Positive'Image (R1.Y));

   Put ("Array type:   ");
   for V of A1 loop
      Put (Integer'Image (V) & " ");
   end loop;
   New_Line;
end Use_Defaults;

As we see in the Use_Defaults procedure, all variables still have their default values, since we haven't assigned any value to them.

In the Ada Reference Manual

Deferred Constants

Deferred constants are declarations where the value of the constant is not specified immediately, but rather deferred to a later point. In that sense, if a constant declaration is deferred, it is actually declared twice:

  1. in the deferred constant declaration, and

  2. in the full constant declaration.

The simplest form of deferred constant is the one that has a full constant declaration in the private part of the package specification. For example:

    
    
    
        
package Deferred_Constants is

   type Speed is new Long_Float;

   Light : constant Speed;
   --      ^ deferred constant declaration

private

   Light : constant Speed := 299_792_458.0;
   --      ^ full constant declaration

end Deferred_Constants;

Another form of deferred constant is the one that imports a constant from an external implementation — using the Import keyword. We can use this to import a constant declaration from an implementation in C. For example, we can declare the light constant in a C file:

    
    
    
        
double light = 299792458.0;

Then, we can import this constant in the Deferred_Constants package:

    
    
    
        
package Deferred_Constants is

   type Speed is new Long_Float;

   Light : constant Speed with
     Import, Convention => C;
   --  ^^^^ deferred constant
   --       declaration; imported
   --       from C file

end Deferred_Constants;

In this case, we don't have a full declaration in the Deferred_Constants package, as the Light constant is imported from the constants.c file.

As a rule, the deferred and the full declarations should match — except, of course, for the actual value that is missing in the deferred declaration. For instance, we're not allowed to use different types in both declarations. However, we may use a subtype in the full declaration — as long as it's compatible with the type that was used in the deferred declaration. For example:

    
    
    
        
package Deferred_Constants is

   type Speed is new Long_Float;

   subtype Positive_Speed is
     Speed range 0.0 .. Speed'Last;

   Light : constant Speed;
   --      ^ deferred constant declaration

private

   Light : constant Positive_Speed :=
             299_792_458.0;
   --      ^ full constant declaration
   --        using a subtype

end Deferred_Constants;

Here, we're using the Speed type in the deferred declaration of the Light constant, but we're using the Positive_Speed subtype in the full declaration.

A useful application of deferred constants is when the value of the constant is calculated using entities not meant to be compile-time visible to clients. As such, these other entities are only visible in the private part of the package, so that's where the value of the deferred constant must be computed. For example, the full constant declaration may be computed by a call to an expression function:

    
    
    
        
package Deferred_Constants is

   type Speed is new Long_Float;

   Light : constant Speed;
   --      ^ deferred constant declaration

private

   function Calculate_Light return Speed is
     (299_792_458.0);

   Light : constant Speed := Calculate_Light;
   --      ^ full constant declaration
   --        calling a private function

end Deferred_Constants;

Here, we call the Calculate_Light function — declared in the private part of the Deferred_Constants package — for the full declaration of the Light constant.

In the Ada Reference Manual

User-defined literals

Note

This feature was introduced in Ada 2022.

Any type definition has a kind of literal associated with it. For example, integer types are associated with integer literals. Therefore, we can initialize an object of integer type with an integer literal:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;

procedure Simple_Integer_Literal is
   V : Integer;
begin
   V := 10;

   Put_Line (Integer'Image (V));
end Simple_Integer_Literal;

Here, 10 is the integer literal that we use to initialize the integer variable V. Other examples of literals are real literals and string literals, as we'll see later.

When we declare an enumeration type, we limit the set of literals that we can use to initialize objects of that type:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;

procedure Simple_Enumeration is
   type Activation_State is (Unknown, Off, On);

   S : Activation_State;
begin
   S := On;
   Put_Line (Activation_State'Image (S));
end Simple_Enumeration;

For objects of Activation_State type, such as S, the only possible literals that we can use are Unknown, Off and On. In this sense, types have a constrained set of literals that can be used for objects of that type.

User-defined literals allow us to extend this set of literals. We could, for example, extend the type declaration of Activation_State and allow the use of integer literals for objects of that type. In this case, we need to use the Integer_Literal aspect and specify a function that implements the conversion from literals to the type we're declaring. For this conversion from integer literals to the Activation_State type, we could specify that 0 corresponds to Off, 1 corresponds to On and other values correspond to Unknown. We'll see the corresponding implementation later.

These are the three kinds of literals and their corresponding aspect:

Literal

Example

Aspect

Integer

1

Integer_Literal

Real

1.0

Real_Literal

String

"On"

String_Literal

For our previous Activation_States type, we could declare a function Integer_To_Activation_State that converts integer literals to one of the enumeration literals that we've specified for the Activation_States type:

    
    
    
        
package Activation_States is

   type Activation_State is (Unknown, Off, On)
     with Integer_Literal =>
            Integer_To_Activation_State;

   function Integer_To_Activation_State
     (S : String)
      return Activation_State;

end Activation_States;

Based on this specification, we can now use an integer literal to initialize an object S of Activation_State type:

S : Activation_State := 1;

Note that we have a string parameter in the declaration of the Integer_To_Activation_State function, even though the function itself is only used to convert integer literals (but not string literals) to the Activation_State type. It's our job to process that string parameter in the implementation of the Integer_To_Activation_State function and convert it to an integer value — using Integer'Value, for example:

    
    
    
        
package body Activation_States is

   function Integer_To_Activation_State
     (S : String)
      return Activation_State is
   begin
      case Integer'Value (S) is
         when 0      => return Off;
         when 1      => return On;
         when others => return Unknown;
      end case;
   end Integer_To_Activation_State;

end Activation_States;

Let's look at a complete example that makes use of all three kinds of literals:

    
    
    
        
package Activation_States is

   type Activation_State is (Unknown, Off, On)
     with String_Literal  =>
            To_Activation_State,
          Integer_Literal =>
            Integer_To_Activation_State,
          Real_Literal    =>
            Real_To_Activation_State;

   function To_Activation_State
     (S : Wide_Wide_String)
      return Activation_State;

   function Integer_To_Activation_State
     (S : String)
      return Activation_State;

   function Real_To_Activation_State
     (S : String)
      return Activation_State;

end Activation_States;


    
        
package body Activation_States is

   function To_Activation_State
     (S : Wide_Wide_String)
      return Activation_State
   is
   begin
      if S = "Off" then
         return Off;
      elsif S = "On" then
         return On;
      else
         return Unknown;
      end if;
   end To_Activation_State;

   function Integer_To_Activation_State
     (S : String)
      return Activation_State
   is
   begin
      case Integer'Value (S) is
         when 0      => return Off;
         when 1      => return On;
         when others => return Unknown;
      end case;
   end Integer_To_Activation_State;

   function Real_To_Activation_State
     (S : String)
      return Activation_State
   is
      V : constant Float := Float'Value (S);
   begin
      if V < 0.0 then
         return Unknown;
      elsif V < 1.0 then
         return Off;
      else
         return On;
      end if;
   end Real_To_Activation_State;

end Activation_States;


    
        
with Ada.Text_IO;       use Ada.Text_IO;
with Activation_States; use Activation_States;

procedure Activation_Examples is
   S : Activation_State;
begin
   S := "Off";
   Put_Line ("String: Off  => "
             & Activation_State'Image (S));

   S := 1;
   Put_Line ("Integer: 1   => "
             & Activation_State'Image (S));

   S := 1.5;
   Put_Line ("Real:    1.5 => "
             & Activation_State'Image (S));
end Activation_Examples;

In this example, we're extending the declaration of the Activation_State type to include string and real literals. For string literals, we use the To_Activation_State function, which converts:

  • the "Off" string to Off,

  • the "On" string to On, and

  • any other string to Unknown.

For real literals, we use the Real_To_Activation_State function, which converts:

  • any negative number to Unknown,

  • a value in the interval [0, 1) to Off, and

  • a value equal or above 1.0 to On.

Note that the string parameter of To_Activation_State function — which converts string literals — is of Wide_Wide_String type, and not of String type, as it's the case for the other conversion functions.

In the Activation_Examples procedure, we show how we can initialize an object of Activation_State type with all kinds of literals (string, integer and real literals).

With the definition of the Activation_State type that we've seen in the complete example, we can initialize an object of this type with an enumeration literal or a string, as both forms are defined in the type specification:

    
    
    
        
with Ada.Text_IO;       use Ada.Text_IO;
with Activation_States; use Activation_States;

procedure Using_String_Literal is
   S1 : constant Activation_State := On;
   S2 : constant Activation_State := "On";
begin
   Put_Line (Activation_State'Image (S1));
   Put_Line (Activation_State'Image (S2));
end Using_String_Literal;

Note we need to be very careful when designing conversion functions. For example, the use of string literals may limit the kind of checks that we can do. Consider the following misspelling of the Off literal:

    
    
    
        
with Ada.Text_IO;       use Ada.Text_IO;
with Activation_States; use Activation_States;

procedure Misspelling_Example is
   S : constant Activation_State :=
         Offf;
   --    ^ Error: Off is misspelled.
begin
   Put_Line (Activation_State'Image (S));
end Misspelling_Example;

As expected, the compiler detects this error. However, this error is accepted when using the corresponding string literal:

    
    
    
        
with Ada.Text_IO;       use Ada.Text_IO;
with Activation_States; use Activation_States;

procedure Misspelling_Example is
   S : constant Activation_State :=
         "Offf";
   --     ^ Error: Off is misspelled.
begin
   Put_Line (Activation_State'Image (S));
end Misspelling_Example;

Here, our implementation of To_Activation_State simply returns Unknown. In some cases, this might be exactly the behavior that we want. However, let's assume that we'd prefer better error handling instead. In this case, we could change the implementation of To_Activation_State to check all literals that we want to allow, and indicate an error otherwise — by raising an exception, for example. Alternatively, we could specify this in the preconditions of the conversion function:

function To_Activation_State
  (S : Wide_Wide_String)
   return Activation_State
     with Pre => S = "Off"  or
                 S = "On"   or
                 S = "Unknown";

In this case, the precondition explicitly indicates which string literals are allowed for the To_Activation_State type.

User-defined literals can also be used for more complex types, such as records. For example:

    
    
    
        
package Silly_Records is

   type Silly is record
      X : Integer;
      Y : Float;
   end record
     with String_Literal => To_Silly;

   function To_Silly (S : Wide_Wide_String)
                      return Silly;
end Silly_Records;


    
        
package body Silly_Records is

   function To_Silly (S : Wide_Wide_String)
                      return Silly
   is
   begin
      if S = "Magic" then
         return (X => 42, Y => 42.0);
      else
         return (X => 0, Y => 0.0);
      end if;
   end To_Silly;

end Silly_Records;


    
        
with Ada.Text_IO;   use Ada.Text_IO;
with Silly_Records; use Silly_Records;

procedure Silly_Magic is
   R1 : Silly;
begin
   R1 := "Magic";
   Put_Line (R1.X'Image & ", " & R1.Y'Image);
end Silly_Magic;

In this example, when we initialize an object of Silly type with a string, its components are:

  • set to 42 when using the "Magic" string; or

  • simply set to zero when using any other string.

Obviously, this example isn't particularly useful. However, the goal is to show that this approach is useful for more complex types where a string literal (or a numeric literal) might simplify handling those types. Used-defined literals let you design types in ways that, otherwise, would only be possible when using a preprocessor or a domain-specific language.

In the Ada Reference Manual