Aggregates

Container Aggregates

Note

This feature was introduced in Ada 2022.

A container aggregate is a list of elements — such as [1, 2, 3] — that we use to initialize or assign to a container. For example:

    
    
    
        
pragma Ada_2022;

with Ada.Containers.Vectors;

procedure Show_Container_Aggregate is

   package Float_Vec is new
     Ada.Containers.Vectors (Positive, Float);

   V : constant Float_Vec.Vector :=
         [1.0, 2.0, 3.0];

   pragma Unreferenced (V);
begin
   null;
end Show_Container_Aggregate;

In this example, [1.0, 2.0, 3.0] is a container aggregate that we use to initialize a vector V.

We can specify container aggregates in three forms:

  • as a null container aggregate, which indicates a container without any elements and is represented by the [] syntax;

  • as a positional container aggregate, where the elements are simply listed in a sequence (such as [1, 2]);

  • as a named container aggregate, where a key is indicated for each element of the list (such as [1 => 10, 2 => 15]).

Let's look at a complete example:

    
    
    
        
pragma Ada_2022;

with Ada.Containers.Vectors;

procedure Show_Container_Aggregate is

   package Float_Vec is new
     Ada.Containers.Vectors (Positive, Float);

   --  Null container aggregate
   Null_V  : constant Float_Vec.Vector :=
               [];

   --  Positional container aggregate
   Pos_V   : constant Float_Vec.Vector :=
               [1.0, 2.0, 3.0];

   --  Named container aggregate
   Named_V : constant Float_Vec.Vector :=
               [1 => 1.0,
                2 => 2.0,
                3 => 3.0];

   pragma Unreferenced (Null_V, Pos_V, Named_V);
begin
   null;
end Show_Container_Aggregate;

In this example, we see the three forms of container aggregates. The difference between positional and named container aggregates is that:

  • for positional container aggregates, the vector index is implied by its position;

while

  • for named container aggregates, the index (or key) of each element is explicitly indicated.

Also, the named container aggregate in this example (Named_V) is using an index as the name (i.e. it's an indexed aggregate). Another option is to use non-indexed aggregates, where we use actual keys — as we do in maps. For example:

    
    
    
        
pragma Ada_2022;

with Ada.Containers.Vectors;
with Ada.Containers.Indefinite_Hashed_Maps;
with Ada.Strings.Hash;

procedure Show_Named_Container_Aggregate is

   package Float_Vec is new
     Ada.Containers.Vectors (Positive, Float);

   package Float_Hashed_Maps is new
     Ada.Containers.Indefinite_Hashed_Maps
       (Key_Type        => String,
        Element_Type    => Float,
        Hash            => Ada.Strings.Hash,
        Equivalent_Keys => "=");

   --  Named container aggregate
   --  using an index
   Indexed_Named_V : constant Float_Vec.Vector :=
                       [1 => 1.0,
                        2 => 2.0,
                        3 => 3.0];

   --  Named container aggregate
   --  using a key
   Keyed_Named_V : constant
     Float_Hashed_Maps.Map :=
       ["Key_1" => 1.0,
        "Key_2" => 2.0,
        "Key_3" => 3.0];

   pragma Unreferenced (Indexed_Named_V,
                        Keyed_Named_V);
begin
   null;
end Show_Named_Container_Aggregate;

In this example, Indexed_Named_V and Keyed_Named_V are both initialized with a named container aggregate. However:

  • the container aggregate for Indexed_Named_V is an indexed aggregate, so we use an index for each element;

while

  • the container aggregate for Keyed_Named_V has a key for each element.

Later on, we'll talk about the Aggregate aspect, which allows for defining custom container aggregates for any record type.

In the Ada Reference Manual

Record aggregates

We've already seen record aggregates in the Introduction to Ada course, so this is just a brief overview on the topic.

As we already know, record aggregates can have positional and named component associations. For example, consider this package:

    
    
    
        
package Points is

   type Point_3D is record
      X, Y, Z : Integer;
   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_3D) is
   begin
      Put_Line ("(X => "
                & Integer'Image (P.X)
                & ",");
      Put_Line (" Y => "
                & Integer'Image (P.Y)
                & ",");
      Put_Line (" Z => "
                & Integer'Image (P.Z)
                & ")");
   end Display;

end Points;

We can use positional or named record aggregates when assigning to an object P of Point_3D type:

    
    
    
        
with Points; use Points;

procedure Show_Record_Aggregates is
   P : Point_3D;
begin
   --  Positional component association
   P := (0, 1, 2);

   Display (P);

   --  Named component association
   P := (X => 3,
         Y => 4,
         Z => 5);

   Display (P);
end Show_Record_Aggregates;

Also, we can have a mixture of both:

    
    
    
        
with Points; use Points;

procedure Show_Record_Aggregates is
   P : Point_3D;
begin
   --  Positional and named component associations
   P := (3, 4,
         Z => 5);

   Display (P);
end Show_Record_Aggregates;

In this case, only the Z component has a named association, while the other components have a positional association.

Note that a positional association cannot follow a named association, so we cannot write P := (3, Y => 4, 5);, for example. Once we start using a named association for a component, we have to continue using it for the remaining components.

In addition, we can choose multiple components at once and assign the same value to them. For that, we use the | syntax:

    
    
    
        
with Points; use Points;

procedure Show_Record_Aggregates is
   P : Point_3D;
begin
   --  Multiple component selection
   P := (X | Y => 5,
         Z     => 6);

   Display (P);
end Show_Record_Aggregates;

Here, we assign 5 to both X and Y.

In the Ada Reference Manual

<>

We can use the <> syntax to tell the compiler to use the default value for specific components. However, if there's no default value for specific components, that component isn't initialized to a known value. For example:

    
    
    
        
with Points; use Points;

procedure Show_Record_Aggregates is
   P : Point_3D;
begin
   P := (0, 1, 2);
   Display (P);

   --  Specifying X component.
   P := (X => 42,
         Y => <>,
         Z => <>);
   Display (P);

   --  Specifying Y and Z components.
   P := (X => <>,
         Y => 10,
         Z => 20);
   Display (P);
end Show_Record_Aggregates;

Here, as the components of Point_3D don't have a default value, those components that have <> are not initialized:

  • when we write (X => 42, Y => <>, Z => <>), only X is initialized;

  • when we write (X => <>, Y => 10, Z => 20) instead, only X is uninitialized.

For further reading...

As we've just seen, all components that get a <> are uninitialized because the components of Point_3D don't have a default value. As no initialization is taking place for those components of the aggregate, the actual value that is assigned to the record is undefined. In other words, the resulting behavior might dependent on the compiler's implementation.

When using GNAT, writing (X => 42, Y => <>, Z => <>) keeps the value of Y and Z intact, while (X => <>, Y => 10, Z => 20) keeps the value of X intact.

If the components of Point_3D had default values, those would have been used. For example, we may change the type declaration of Point_3D and use default values for each component:

    
    
    
        
package Points is

   type Point_3D is record
      X : Integer := 10;
      Y : Integer := 20;
      Z : Integer := 30;
   end record;

   procedure Display (P : Point_3D);

end Points;

Then, writing <> makes use of those default values we've just specified:

    
    
    
        
with Points; use Points;

procedure Show_Record_Aggregates is
   P : Point_3D := (0, 0, 0);
begin
   --  Using default value for
   --  all components
   P := (X => <>,
         Y => <>,
         Z => <>);
   Display (P);
end Show_Record_Aggregates;

Now, as expected, the default values of each component (10, 20 and 30) are used when we write <>.

Similarly, we can specify a default value for the type of each component. For example, let's declare a Point_Value type with a default value — using the Default_Value aspect — and use it in the Point_3D record type:

    
    
    
        
package Points is

   type Point_Value is new Float
     with Default_Value => 99.9;

   type Point_3D is record
      X : Point_Value;
      Y : Point_Value;
      Z : Point_Value;
   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_3D) is
   begin
      Put_Line ("(X => "
                & Point_Value'Image (P.X)
                & ",");
      Put_Line (" Y => "
                & Point_Value'Image (P.Y)
                & ",");
      Put_Line (" Z => "
                & Point_Value'Image (P.Z)
                & ")");
   end Display;

end Points;

Then, writing <> makes use of the default value of the Point_Value type:

    
    
    
        
with Points; use Points;

procedure Show_Record_Aggregates is
   P : Point_3D := (0.0, 0.0, 0.0);
begin
   --  Using default value of Point_Value
   --  for all components
   P := (X => <>,
         Y => <>,
         Z => <>);
   Display (P);
end Show_Record_Aggregates;

In this case, the default value of the Point_Value type (99.9) is used for all components when we write <>.

others

Also, we can use the others selector to assign a value to all components that aren't explicitly mentioned in the aggregate. For example:

    
    
    
        
with Points; use Points;

procedure Show_Record_Aggregates is
   P : Point_3D;
begin
   --  Specifying X component;
   --  using 42 for all
   --  other components.
   P := (X      => 42,
         others => 100);
   Display (P);

   --  Specifying all components
   P := (others => 256);
   Display (P);
end Show_Record_Aggregates;

When we write P := (X => 42, others => 100), we're assigning 42 to X and 100 to all other components (Y and Z in this case). Also, when we write P := (others => 256), all components have the same value (256).

Note that writing a specific value in others — such as (others => 256) — only works when all components have the same type. In this example, all components of Point_3D have the same type: Integer. If we had components with different types in the components selected by others, say Integer and Float, then (others => 256) would trigger a compilation error. For example, consider this package:

    
    
    
        
package Custom_Records is

   type Integer_Float is record
     A, B : Integer := 0;
     Y, Z : Float   := 0.0;
   end record;

end Custom_Records;

If we had written an aggregate such as (others => 256) for an object of type Integer_Float, the value (256) would be OK for components A and B, but not for components Y and Z:

    
    
    
        
with Custom_Records; use Custom_Records;

procedure Show_Record_Aggregates_Others is
   Dummy : Integer_Float;
begin
   --  ERROR: components selected by
   --         others must be of same
   --         type.
   Dummy := (others => 256);
end Show_Record_Aggregates_Others;

We can fix this compilation error by making sure that others only refers to components of the same type:

    
    
    
        
with Custom_Records; use Custom_Records;

procedure Show_Record_Aggregates_Others is
   Dummy : Integer_Float;
begin
   --  OK: components selected by
   --      others have Integer type.
   Dummy := (Y | Z  => 256.0,
             others => 256);
end Show_Record_Aggregates_Others;

In any case, writing (others => <>) is always accepted by the compiler because it simply selects the default value of each component, so the type of those values is unambiguous:

    
    
    
        
with Custom_Records; use Custom_Records;

procedure Show_Record_Aggregates_Others is
   Dummy : Integer_Float;
begin
   Dummy := (others => <>);
end Show_Record_Aggregates_Others;

This code compiles because <> uses the appropriate default value of each component.

Record discriminants

When a record type has discriminants, they must appear as components of an aggregate of that type. For example, consider this package:

    
    
    
        
package Points is

   type Point_Dimension is (Dim_1, Dim_2, Dim_3);

   type Point (D : Point_Dimension) is record
      case D is
      when Dim_1 =>
         X1         : Integer;
      when Dim_2 =>
         X2, Y2     : Integer;
      when Dim_3 =>
         X3, Y3, Z3 : Integer;
      end case;
   end record;

   procedure Display (P : Point);

end Points;

    
        
with Ada.Text_IO; use Ada.Text_IO;

package body Points is

   procedure Display (P : Point) is
   begin
      Put_Line (Point_Dimension'Image (P.D));

      case P.D is
      when Dim_1 =>
         Put_Line ("  (X => "
                   & Integer'Image (P.X1)
                   & ")");
      when Dim_2 =>
         Put_Line ("  (X => "
                   & Integer'Image (P.X2)
                   & ",");
         Put_Line ("   Y => "
                   & Integer'Image (P.Y2)
                  & ")");
      when Dim_3 =>
         Put_Line ("  (X => "
                   & Integer'Image (P.X3)
                   & ",");
         Put_Line ("   Y => "
                   & Integer'Image (P.Y3)
                   & ",");
         Put_Line ("   Z => "
                   & Integer'Image (P.Z3)
                   & ")");
      end case;
   end Display;

end Points;

To write aggregates of the Point type, we have to specify the D discriminant as a component of the aggregate. The discriminant must be included in the aggregate — and must be static — because the compiler must be able to examine the aggregate to determine if it is both complete and consistent. All components must be accounted for one way or another, as usual — but, in addition, references to those components whose existence depends on the discriminant's values must be consistent with the actual discriminant value used in the aggregate. For example, for type Point, an aggregate can only reference the X3, Y3, and Z3 components when Dim_3 is specified for the discriminant D; otherwise, those three components don't exist in that aggregate. Also, the discriminant D must be the first one if we use positional component association. For example:

    
    
    
        
with Points; use Points;

procedure Show_Rec_Aggregate_Discriminant is
   --  Positional component association
   P1 : constant Point := (Dim_1, 0);

   --  Named component association
   P2 : constant Point := (D  => Dim_2,
                           X2 => 3,
                           Y2 => 4);

   --  Positional / named component association
   P3 : constant Point := (Dim_3,
                           X3 => 3,
                           Y3 => 4,
                           Z3 => 5);
begin
   Display (P1);
   Display (P2);
   Display (P3);
end Show_Rec_Aggregate_Discriminant;

As we see in this example, we can use any component association in the aggregate, as long as we make sure that the discriminants of the type appear as components — and are the first components in the case of positional component association.

Null records

A null record is a record that doesn't have any components. Consequently, it cannot store any information. When declaring a null record, we simply write null instead of declaring actual components, as we usually do for records. For example:

    
    
    
        
package Null_Recs is

   type Null_Record is record
      null;
   end record;

end Null_Recs;

Note that the syntax can be simplified to is null record, which is much more common than the previous form:

    
    
    
        
package Null_Recs is

   type Null_Record is null record;

end Null_Recs;

Although a null record doesn't have components, we can still specify subprograms for it. For example, we could specify an addition operation for it:

    
    
    
        
package Null_Recs is

   type Null_Record is null record;

   function "+" (A, B : Null_Record)
                 return Null_Record;

end Null_Recs;

    
        
package body Null_Recs is

   function "+" (A, B : Null_Record)
                 return Null_Record
   is
      pragma Unreferenced (A, B);
   begin
      return (null record);
   end "+";

end Null_Recs;

    
        
with Null_Recs; use Null_Recs;

procedure Show_Null_Rec is
   A, B : Null_Record;
begin
   B := A + A;
   A := A + B;
end Show_Null_Rec;

In the Ada Reference Manual

Simple Prototyping

A null record doesn't provide much functionality on itself, as we're not storing any information in it. However, it's far from being useless. For example, we can make use of null records to design an API, which we can then use in an application without having to implement the actual functionality of the API. This allows us to design a prototype without having to think about all the implementation details of the API in the first stage.

Consider this example:

    
    
    
        
package Devices is

   type Device is private;

   function Create
     (Active : Boolean)
      return Device;

   procedure Reset
     (D : out Device) is null;

   procedure Process
     (D : in out Device) is null;

   procedure Activate
     (D : in out Device) is null;

   procedure Deactivate
     (D : in out Device) is null;

private

   type Device is null record;

   function Create (Active : Boolean)
                    return Device is
     (null record);

end Devices;

    
        
with Ada.Text_IO; use Ada.Text_IO;
with Devices;     use Devices;

procedure Show_Device is
   A : Device;
begin
   Put_Line ("Creating device...");
   A := Create (Active => True);

   Put_Line ("Processing on device...");
   Process (A);

   Put_Line ("Deactivating device...");
   Deactivate (A);

   Put_Line ("Activating device...");
   Activate (A);

   Put_Line ("Resetting device...");
   Reset (A);
end Show_Device;

In the Devices package, we're declaring the Device type and its primitive subprograms: Create, Reset, Process, Activate and Deactivate. This is the API that we use in our prototype. Note that, although the Device type is declared as a private type, it's still defined as a null record in the full view.

In this example, the Create function, implemented as an expression function in the private part, simply returns a null record. As expected, this null record returned by Create matches the definition of the Device type.

All procedures associated with the Device type are implemented as null procedures, which means they don't actually have an implementation nor have any effect. We'll discuss this topic later on in the course.

In the Show_Device procedure — which is an application that implements our prototype —, we declare an object of Device type and call all subprograms associated with that type.

Extending the prototype

Because we're either using expression functions or null procedures in the specification of the Devices package, we don't have a package body for it (as there's nothing to be implemented). We could, however, move those user messages from the Show_Devices procedure to a dummy implementation of the Devices package. This is the adapted code:

    
    
    
        
package Devices is

   type Device is null record;

   function Create (Active : Boolean)
                    return Device;

   procedure Reset (D : out Device);

   procedure Process (D : in out Device);

   procedure Activate (D : in out Device);

   procedure Deactivate (D : in out Device);

end Devices;

    
        
with Ada.Text_IO; use Ada.Text_IO;

package body Devices is

   function Create (Active : Boolean)
                    return Device
   is
      pragma Unreferenced (Active);
   begin
      Put_Line ("Creating device...");
      return (null record);
   end Create;

   procedure Reset (D : out Device)
   is
      pragma Unreferenced (D);
   begin
      Put_Line ("Processing on device...");
   end Reset;

   procedure Process (D : in out Device)
   is
      pragma Unreferenced (D);
   begin
      Put_Line ("Deactivating device...");
   end Process;

   procedure Activate (D : in out Device)
   is
      pragma Unreferenced (D);
   begin
      Put_Line ("Activating device...");
   end Activate;

   procedure Deactivate (D : in out Device)
   is
      pragma Unreferenced (D);
   begin
      Put_Line ("Resetting device...");
   end Deactivate;

end Devices;

    
        
with Devices; use Devices;

procedure Show_Device is
   A : Device;
begin
   A := Create (Active => True);
   Process (A);
   Deactivate (A);
   Activate (A);
   Reset (A);
end Show_Device;

As we changed the specification of the Devices package to not use null procedures, we now need a corresponding package body for it. In this package body, we implement the operations on the Device type, which actually just display a user message indicating which operation is being called.

Let's focus on this updated version of the Show_Device procedure. Now that we've removed all those calls to Put_Line from this procedure and just have the calls to operations associated with the Device type, it becomes more apparent that, even though Device is just a null record, we can design an application with a sequence of various commands operating on it. Also, when we just read the source-code of the Show_Device procedure, there's no clear indication that the Device type doesn't actually hold any information.

More complex applications

As we've just seen, we can use null records like any other type and create complex prototypes with them. We could, for instance, design an application that makes use of many null records, or even have types that depend on or derive from null records. Let's see a simple example:

    
    
    
        
package Many_Devices is

   type Device is null record;

   type Device_Config is null record;

   function Create (Config : Device_Config)
                    return Device is
     (null record);

   type Derived_Device is new Device;

   procedure Process (D : Derived_Device) is null;

end Many_Devices;

    
        
with Many_Devices; use Many_Devices;

procedure Show_Derived_Device is
   A : Device;
   B : Derived_Device;
   C : Device_Config;
begin
   A := Create (Config => C);
   B := Create (Config => C);

   Process (B);
end Show_Derived_Device;

In this example, the Create function has a null record parameter (of Device_Config type) and returns a null record (of Device type). Also, we derive the Derived_Device type from the Device type. Consequently, Derived_Device is also a null record (since it's derived from a null record). In the Show_Derived_Device procedure, we declare objects of those types (A, B and C) and call primitive subprograms to operate on them.

This example shows that, even though the types we've declared are just null records, they can still be used to represent dependencies in our application.

Implementing the API

Let's focus again on the previous example. After we have an initial prototype, we can start implementing some of the functionality needed for the Device type. For example, we can store information about the current activation state in the record:

    
    
    
        
package Devices is

   type Device is private;

   function Create (Active : Boolean)
                    return Device;

   procedure Reset (D : out Device);

   procedure Process (D : in out Device);

   procedure Activate (D : in out Device);

   procedure Deactivate (D : in out Device);

private

   type Device is record
      Active : Boolean;
   end record;

end Devices;

    
        
with Ada.Text_IO; use Ada.Text_IO;

package body Devices is

   function Create (Active : Boolean)
                    return Device
   is
      pragma Unreferenced (Active);
   begin
      Put_Line ("Creating device...");
      return (Active => Active);
   end Create;

   procedure Reset (D : out Device)
   is
      pragma Unreferenced (D);
   begin
      Put_Line ("Processing on device...");
   end Reset;

   procedure Process (D : in out Device)
   is
      pragma Unreferenced (D);
   begin
      Put_Line ("Deactivating device...");
   end Process;

   procedure Activate (D : in out Device)
   is
   begin
      Put_Line ("Activating device...");
      D.Active := True;
   end Activate;

   procedure Deactivate (D : in out Device)
   is
   begin
      Put_Line ("Resetting device...");
      D.Active := False;
   end Deactivate;

end Devices;

    
        
with Ada.Text_IO; use Ada.Text_IO;
with Devices;     use Devices;

procedure Show_Device is
   A : Device;
begin
   A := Create (Active => True);
   Process (A);
   Deactivate (A);
   Activate (A);
   Reset (A);
end Show_Device;

Now, the Device record contains an Active component, which is used in the updated versions of Create, Activate and Deactivate.

Note that we haven't done any change to the implementation of the Show_Device procedure: it's still the same application as before. As we've been hinting in the beginning, using null records makes it easy for us to first create a prototype — as we did in the Show_Device procedure — and postpone the API implementation to a later phase of the project.

Tagged null records

A null record may be tagged, as we can see in this example:

    
    
    
        
package Null_Recs is

   type Tagged_Null_Record is
     tagged null record;

   type Abstract_Tagged_Null_Record is
     abstract tagged null record;

end Null_Recs;

As we see in this example, a type can be tagged, or even abstract tagged. We discuss abstract types later on in the course.

As expected, in addition to deriving from tagged types, we can also extend them. For example:

    
    
    
        
package Devices is

   type Device is private;

   function Create (Active : Boolean)
                    return Device;

   type Derived_Device is private;

private

   type Device is tagged null record;

   function Create (Active : Boolean)
                    return Device is
     (null record);

   type Derived_Device is new Device with record
      Active : Boolean;
   end record;

   function Create (Active : Boolean)
                    return Derived_Device is
     (Active => Active);

end Devices;

In this example, we derive Derived_Device from the Device type and extend it with the Active component. (Because we have a type extension, we also need to override the Create function.)

Since we're now introducing elements from object-oriented programming, we could consider using interfaces instead of null records. We'll discuss this topic later on in the course.

Full coverage rules for Aggregates

Note

This section was originally written by Robert A. Duff and published as Gem #1: Limited Types in Ada 2005.

One interesting feature of Ada are the full coverage rules for aggregates. For example, suppose we have a record type:

    
    
    
        
with Ada.Strings.Unbounded;
use  Ada.Strings.Unbounded;

package Persons is
   type Years is new Natural;

   type Person is record
      Name : Unbounded_String;
      Age  : Years;
   end record;
end Persons;

We can create an object of the type using an aggregate:

    
    
    
        
with Ada.Strings.Unbounded;
use  Ada.Strings.Unbounded;

with Persons; use Persons;

procedure Show_Aggregate_Init is

   X : constant Person :=
         (Name =>
            To_Unbounded_String ("John Doe"),
          Age  => 25);
begin
   null;
end Show_Aggregate_Init;

The full coverage rules say that every component of Person must be accounted for in the aggregate. If we later modify type Person by adding a component:

    
    
    
        
with Ada.Strings.Unbounded;
use  Ada.Strings.Unbounded;

package Persons is
   type Years is new Natural;

   type Person is record
      Name      : Unbounded_String;
      Age       : Natural;
      Shoe_Size : Positive;
   end record;
end Persons;

and we forget to modify X accordingly, the compiler will remind us. Case statements also have full coverage rules, which serve a similar purpose.

Of course, we can defeat the full coverage rules by using others (usually for array aggregates and case statements, but occasionally useful for record aggregates):

    
    
    
        
with Ada.Strings.Unbounded;
use  Ada.Strings.Unbounded;

with Persons; use Persons;

procedure Show_Aggregate_Init_Others is

   X : constant Person :=
         (Name   =>
            To_Unbounded_String ("John Doe"),
          others => 25);
begin
   null;
end Show_Aggregate_Init_Others;

According to the Ada RM, others here means precisely the same thing as Age | Shoe_Size. But that's wrong: what others really means is "all the other components, including the ones we might add next week or next year". That means you shouldn't use others unless you're pretty sure it should apply to all the cases that haven't been invented yet.

Later on, we'll discuss full coverage rules for limited types.

Array aggregates

We've already discussed array aggregates in the Introduction to Ada course. Therefore, this section just presents some details about this topic.

In the Ada Reference Manual

Positional and named array aggregates

Note

The array aggregate syntax using brackets (e.g.: [1, 2, 3]), which we mention in this section, was introduced in Ada 2022.

Similar to record aggregates, array aggregates can be positional or named. Consider this package:

    
    
    
        
package Points is

   type Point_3D is array (1 .. 3) of Integer;

   procedure Display (P : Point_3D);

end Points;

    
        
pragma Ada_2022;

with Ada.Text_IO; use Ada.Text_IO;

package body Points is

   procedure Display (P : Point_3D) is
   begin
      Put_Line ("(X => "
                & Integer'Image (P (1))
                & ",");
      Put_Line (" Y => "
                & Integer'Image (P (2))
                & ",");
      Put_Line (" Z => "
                & Integer'Image (P (3))
                & ")");
   end Display;

end Points;

We can write positional or named aggregates when assigning to an object P of Point_3D type:

    
    
    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Array_Aggregates is
   P : Point_3D;
begin
   --  Positional component association
   P := [0, 1, 2];

   Display (P);

   --  Named component association
   P := [1 => 3,
         2 => 4,
         3 => 5];

   Display (P);
end Show_Array_Aggregates;

In this example, we assign a positional array aggregate ([1, 2, 3]) to P. Then, we assign a named array aggregate ([1 => 3, 2 => 4, 3 => 5]) to P. In this case, the names are the indices of the components we're assigning to.

We can also assign array aggregates to slices:

    
    
    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Array_Aggregates is
   P : Point_3D := [others => 0];
begin
   --  Positional component association
   P (2 .. 3) := [1, 2];

   Display (P);

   --  Named component association
   P (2 .. 3) := [1 => 3,
                  2 => 4];

   Display (P);
end Show_Array_Aggregates;

Note that, when using a named array aggregate, the index (name) that we use in the aggregate doesn't have to match the slice. In this example, we're assigning the component from index 1 of the aggregate to the component of index 2 of the array P (and so on).

Historically

In the first versions of Ada, we could only write array aggregates using parentheses.

    
    
    
        
pragma Ada_2012;

with Points; use Points;

procedure Show_Array_Aggregates is
   P : Point_3D;
begin
   --  Positional component association
   P := (0, 1, 2);

   Display (P);

   --  Named component association
   P := (1 => 3,
         2 => 4,
         3 => 5);

   Display (P);
end Show_Array_Aggregates;

This syntax is considered obsolescent since Ada 2022: brackets ([1, 2, 3]) should be used instead.

Null array aggregate

Note

This feature was introduced in Ada 2022.

We can also write null array aggregates: []. As the name implies, this kind of array aggregate doesn't have any components.

Consider this package:

    
    
    
        
package Integer_Arrays is

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

   procedure Display (A : Integer_Array);

end Integer_Arrays;

    
        
pragma Ada_2022;

with Ada.Text_IO; use Ada.Text_IO;

package body Integer_Arrays is

   procedure Display (A : Integer_Array) is
   begin
      Put_Line ("Length = "
                & A'Length'Image);

      Put_Line ("(");
      for I in A'Range loop
         Put ("  "
              & I'Image
              & " => "
              & A (I)'Image);
         if I /= A'Last then
            Put_Line (",");
         else
            New_Line;
         end if;
      end loop;
      Put_Line (")");
   end Display;

end Integer_Arrays;

We can initialize an object N of Integer_Array type with a null array aggregate:

    
    
    
        
pragma Ada_2022;

with Integer_Arrays; use Integer_Arrays;

procedure Show_Array_Aggregates is
   N : constant Integer_Array := [];
begin
   Display (N);
end Show_Array_Aggregates;

In this example, when we call the Display procedure, we confirm that N doesn't have any components.

|, <>, others

We've seen the following syntactic elements when we were discussing record aggregates: |, <> and others. We can apply them to array aggregates as well:

    
    
    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Array_Aggregates is
   P : Point_3D;
begin
   --  All components have a value of zero.
   P := [others => 0];

   Display (P);

   --  Both first and second components have
   --  a value of three.
   P := [1 | 2 => 3,
         3     => 4];

   Display (P);

   --  The default value is used for the first
   --  component, and all other components
   --  have a value of five.
   P := [1      => <>,
         others => 5];

   Display (P);
end Show_Array_Aggregates;

In this example, we use the |, <> and others elements in a very similar way as we did with record aggregates. (See the comments in the code example for more details.)

Note that, as for record aggregates, the <> makes use of the default value (if it is available). We discuss this topic in more details later on.

..

We can also use the range syntax (..) with array aggregates:

    
    
    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Array_Aggregates is
   P : Point_3D;
begin
   --  All components have a value of zero.
   P := [1 .. 3 => 0];

   Display (P);

   --  Both first and second components have
   --  a value of three.
   P := [1 .. 2 => 3,
         3      => 4];

   Display (P);

   --  The default value is used for the first
   --  component, and all other components
   --  have a value of five.
   P := [1      => <>,
         2 .. 3 => 5];

   Display (P);
end Show_Array_Aggregates;

This example is a variation of the previous one. However, in this case, we're using ranges instead of the | and others syntax.

Missing components

All aggregate components must have an associated value. If we don't specify a value for a certain component, an exception is raised:

    
    
    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Array_Aggregates is
   P : Point_3D;
begin
   P := [1 => 4];
   --  ERROR: value of components at indices
   --         2 and 3 are missing

   Display (P);
end Show_Array_Aggregates;

We can use others to specify a value to all components that haven't been explicitly mentioned in the aggregate:

    
    
    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Array_Aggregates is
   P : Point_3D;
begin
   P := [1 => 4, others => 0];
   --  OK: unspecified components have a
   --      value of zero

   Display (P);
end Show_Array_Aggregates;

However, others can only be used when the range is known — compilation fails otherwise:

    
    
    
        
pragma Ada_2022;

with Integer_Arrays; use Integer_Arrays;

procedure Show_Array_Aggregates is
   N1 : Integer_Array := [others => 0];
   --  ERROR: range is unknown

   N2 : Integer_Array (1 .. 3) := [others => 0];
   --  OK: range is known
begin
   Display (N1);
   Display (N2);
end Show_Array_Aggregates;

Of course, we could fix the declaration of N1 by specifying a range — e.g. N1 : Integer_Array (1 .. 10) := [others => 0];.

Iterated component association

Note

This feature was introduced in Ada 2022.

We can use an iterated component association to specify an aggregate. This is the general syntax:

--  All components have a value of zero
P := [for I in 1 .. 3 => 0];

Let's see a complete example:

    
    
    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Array_Aggregates is
   P : Point_3D;
begin
   --  All components have a value of zero
   P := [for I in 1 .. 3 => 0];

   Display (P);

   --  Both first and second components have
   --  a value of three
   P := [for I in 1 .. 3 =>
           (if I = 1 or I = 2
            then 3
            else 4)];

   Display (P);

   --  The first component has a value of 99
   --  and all other components have a value
   --  that corresponds to its index
   P := [1 => 99,
         for I in 2 .. 3 => I];

   Display (P);
end Show_Array_Aggregates;

In this example, we use iterated component associations in different ways:

  1. We write a simple iteration ([for I in 1 .. 3 => 0]).

  2. We use a conditional expression in the iteration: [for I in 1 .. 3 => (if I = 1 or I = 2 then 3 else 4)].

  3. We use a named association for the first element, and then iterated component association for the remaining components: [1 => 99, for I in 2 .. 3 => I].

So far, we've used a discrete choice list (in the for I in Range form) in the iterated component association. We could use an iterator (in the for E of form) instead. For example:

    
    
    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Array_Aggregates is
   P : Point_3D := [for I in Point_3D'Range => I];
begin
   --  Each component is doubled
   P := [for E of P => E * 2];

   Display (P);

   --  Each component is increased
   --  by one
   P := [for E of P => E + 1];

   Display (P);
end Show_Array_Aggregates;

In this example, we use iterators in different ways:

  1. We write [for E of P => E * 2] to double the value of each component.

  2. We write [for E of P => E + 1] to increase the value of each component by one.

Of course, we could write more complex operations on E in the iterators.

Multidimensional array aggregates

So far, we've discussed one-dimensional array aggregates. We can also use the same constructs when dealing with multidimensional arrays. Consider, for example, this package:

    
    
    
        
package Matrices is

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

   procedure Display (M : Matrix);

end Matrices;

    
        
pragma Ada_2022;

with Ada.Text_IO; use Ada.Text_IO;

package body Matrices is

   procedure Display (M : Matrix) is

      procedure Display_Row (M : Matrix;
                             I : Integer) is
      begin
         Put_Line ("  (");
         for J in M'Range (2) loop
            Put ("    "
                 & J'Image
                 & " => "
                 & M (I, J)'Image);
            if J /= M'Last (2) then
               Put_Line (",");
            else
               New_Line;
            end if;
         end loop;
         Put ("  )");
      end Display_Row;

   begin
      Put_Line ("Length (1) = "
                & M'Length (1)'Image);
      Put_Line ("Length (2) = "
                & M'Length (2)'Image);

      Put_Line ("(");
      for I in M'Range (1) loop
         Display_Row (M, I);
         if I /= M'Last (1) then
            Put_Line (",");
         else
            New_Line;
         end if;
      end loop;
      Put_Line (")");

   end Display;

end Matrices;

We can assign multidimensional aggregates to a matrix M using positional or named component association:

    
    
    
        
pragma Ada_2022;

with Matrices; use Matrices;

procedure Show_Array_Aggregates is
   M : Matrix (1 .. 2, 1 .. 3);
begin
   --  Positional component association
   M := [[0, 1, 2],
         [3, 4, 5]];

   Display (M);

   --  Named component association
   M := [[1 => 3,
          2 => 4,
          3 => 5],
         [1 => 6,
          2 => 7,
          3 => 8]];

   Display (M);

end Show_Array_Aggregates;

The first aggregate we use in this example is [[0, 1, 2], [3, 4, 5]]. Here, [0, 1, 2] and [3, 4, 5] are subaggregates of the multidimensional aggregate. Subaggregates don't have a type themselves, but are rather just considered part of a multidimensional aggregate (which, of course, has an array type). In this sense, a subaggregate such as [0, 1, 2] is different from a one-dimensional aggregate (such as [0, 1, 2]), even though they are written in the same way.

Strings in subaggregates

In the case of matrices using characters, we can use strings in the corresponding array aggregates. Consider this package:

    
    
    
        
package String_Lists is

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

   procedure Display (SL : String_List);

end String_Lists;

    
        
pragma Ada_2022;

with Ada.Text_IO; use Ada.Text_IO;

package body String_Lists is

   procedure Display (SL : String_List) is

      procedure Display_Row (SL : String_List;
                             I  : Integer) is
      begin
         Put ("  (");
         for J in SL'Range (2) loop
            Put (SL (I, J));
         end loop;
         Put (")");
      end Display_Row;

   begin
      Put_Line ("Length (1) = "
                & SL'Length (1)'Image);
      Put_Line ("Length (2) = "
                & SL'Length (2)'Image);

      Put_Line ("(");
      for I in SL'Range (1) loop
         Display_Row (SL, I);
         if I /= SL'Last (1) then
            Put_Line (",");
         else
            New_Line;
         end if;
      end loop;
      Put_Line (")");
   end Display;

end String_Lists;

Then, when assigning to an object SL of String_List type, we can use strings in the aggregates:

    
    
    
        
pragma Ada_2022;

with String_Lists; use String_Lists;

procedure Show_Array_Aggregates is
   SL : String_List (1 .. 2, 1 .. 3);
begin
   --  Positional component association
   SL := ["ABC",
          "DEF"];

   Display (SL);

   --  Named component associations
   SL := [[1 => 'A',
           2 => 'B',
           3 => 'C'],
          [1 => 'D',
           2 => 'E',
           3 => 'F']];

   Display (SL);

   SL := [[1 => 'X',
           2 => 'Y',
           3 => 'Z'],
          [others => ' ']];

   Display (SL);
end Show_Array_Aggregates;

In the first assignment to SL, we have the aggregate ["ABC", "DEF"], which uses strings as subaggregates. (Of course, we can use a named aggregate and assign characters to the individual components.)

<> and default values

As we indicated earlier, the <> syntax sets a component to its default value — if such a default value is available. If a default value isn't defined, however, the component will remain uninitialized, so that the behavior is undefined. Let's look at more complex example to illustrate this situation. Consider this package, for example:

    
    
    
        
package Points is

   subtype Point_Value is Integer;

   type Point_3D is record
      X, Y, Z : Point_Value;
   end record;

   procedure Display (P : Point_3D);

   type Point_3D_Array is
     array (Positive range <>) of Point_3D;

   procedure Display (PA : Point_3D_Array);

end Points;

    
        
with Ada.Text_IO; use Ada.Text_IO;

package body Points is

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

   procedure Display (PA : Point_3D_Array) is
   begin
      Put_Line ("(");
      for I in PA'Range (1) loop
         Put_Line ("  "
                   & Integer'Image (I)
                   & " =>");
         Display (PA (I));
         if I /= PA'Last (1) then
            Put_Line (",");
         else
            New_Line;
         end if;
      end loop;
      Put_Line (")");
   end Display;

end Points;

Then, let's use <> for the array components:

    
    
    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Record_Aggregates is
   PA : Point_3D_Array (1 .. 2);
begin
   PA := [ (X => 3,
            Y => 4,
            Z => 5),
           (X => 6,
            Y => 7,
            Z => 8) ];
   Display (PA);

   --  Array components are
   --  uninitialized.
   PA := [1 => <>,
          2 => <>];
   Display (PA);
end Show_Record_Aggregates;

Because the record components (of the Point_3D type) don't have default values, they remain uninitialized when we write [1 => <>, 2 => <>]. (In fact, you may see garbage in the values displayed by the Display procedure.)

When a default value is specified, it is used whenever <> is specified. For example, we could use a type that has the Default_Value aspect in its specification:

    
    
    
        
package Integer_Arrays is

   type Value is new Integer
     with Default_Value => 99;

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

   procedure Display (A : Integer_Array);

end Integer_Arrays;

    
        
pragma Ada_2022;

with Integer_Arrays; use Integer_Arrays;

procedure Show_Array_Aggregates is
   N : Integer_Array (1 .. 4);
begin
   N := [for I in N'Range => Value (I)];
   Display (N);

   N := [others => <>];
   Display (N);
end Show_Array_Aggregates;

When writing an aggregate for the Point_3D type, any component that has <> gets the default value of the Point type (99):

For further reading...

Similarly, we could specify the Default_Component_Value aspect (which we discussed earlier on) in the declaration of the array type:

    
    
    
        
package Integer_Arrays is

   type Value is new Integer;

   type Integer_Array is
     array (Positive range <>) of Value
       with Default_Component_Value => 9999;

   procedure Display (A : Integer_Array);

end Integer_Arrays;

    
        
pragma Ada_2022;

with Integer_Arrays; use Integer_Arrays;

procedure Show_Array_Aggregates is
   N : Integer_Array (1 .. 4);
begin
   N := [for I in N'Range => Value (I)];
   Display (N);

   N := [others => <>];
   Display (N);
end Show_Array_Aggregates;

In this case, when writing <> for a component, the value specified in the Default_Component_Value aspect is used.

Finally, we might want to use both Default_Value (which we discussed previously) and Default_Component_Value aspects at the same time. In this case, the value specified in the Default_Component_Value aspect has higher priority:

    
    
    
        
package Integer_Arrays is

   type Value is new Integer
     with Default_Value => 99;

   type Integer_Array is
     array (Positive range <>) of Value
       with Default_Component_Value => 9999;

   procedure Display (A : Integer_Array);

end Integer_Arrays;

    
        
pragma Ada_2022;

with Integer_Arrays; use Integer_Arrays;

procedure Show_Array_Aggregates is
   N : Integer_Array (1 .. 4);
begin
   N := [for I in N'Range => Value (I)];
   Display (N);

   N := [others => <>];
   Display (N);
end Show_Array_Aggregates;

Here, 9999 is used when we specify <> for a component.

Extension Aggregates

Extension aggregates provide a convenient way to express an aggregate for a type that extends — adds components to — some existing type (the "ancestor"). Although mainly a matter of convenience, an extension aggregate is essential when we want to express an aggregate for an extension of a private ancestor type, that is, when we don't have compile-time visibility to the ancestor type's components.

In the Ada Reference Manual

Assignments to objects of derived types

Before we discuss extension aggregates in more detail, though, let's start with a simple use-case. Let's say we have:

  • an object A of tagged type T1, and

  • an object B of tagged type T2, which extends T1.

We can initialize object B by:

  • copying the T1 specific information from A to B, and

  • initializing the T2 specific components of B.

We can translate the description above to the following code:

   A : T1;
   B : T2;
begin
   T1 (B) := A;

   B.Extended_Component_1 := Some_Value;
   --  [...]

Here, we use T1 (B) to select the ancestor view of object B, and we copy all the information from A to this part of B. Then, we initialize the remaining components of B. We'll elaborate on this kind of assignments later on.

Example: Points

To present a more concrete example, let's start with a package that defines one, two and three-dimensional point types:

    
    
    
        
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;

Let's now focus on the Show_Points procedure below, where we initialize a two-dimensional point using a one-dimensional point.

    
    
    
        
with Points; use Points;

procedure Show_Points is
   P_1D : Point_1D;
   P_2D : Point_2D;
begin
   P_1D := (X => 0.5);
   Display (P_1D);

   Point_1D (P_2D) := P_1D;
   --  Equivalent to: "P_2D.X := P_1D.X;"

   P_2D.Y := 0.7;

   Display (P_2D);
end Show_Points;

In this example, we're initializing P_2D using the information stored in P_1D. By writing Point_1D (P_2D) on the left side of the assignment, we specify that we want to limit our focus on the Point_1D view of the P_2D object. Then, we assign P_1D to the Point_1D view of the P_2D object. This assignment initializes the X component of the P_2D object. The Point_2D specific components are not changed by this assignment. (In other words, this is equivalent to just writing P_2D.X := P_1D.X, as the Point_1D type only has the X component.) Finally, in the next line, we initialize the Y component with 0.7.

Using extension aggregates

Note that, in the assignment to P_1D, we use a record aggregate. Extension aggregates are similar to record aggregates, but they include the with keyword — for example: (Obj1 with Y => 0.5). This allows us to assign to an object with information from another object Obj1 of a parent type and, in the same expression, set the value of the Y component of the type extension.

Let's rewrite the previous Show_Points procedure using extension aggregates:

    
    
    
        
with Points; use Points;

procedure Show_Points is
   P_1D : Point_1D;
   P_2D : Point_2D;
begin
   P_1D := (X => 0.5);
   Display (P_1D);

   P_2D := (P_1D with Y => 0.7);
   Display (P_2D);
end Show_Points;

When we write P_2D := (P_1D with Y => 0.7), we're initializing P_2D using:

  • the information from the P_1D object — of Point_1D type, which is an ancestor of the Point_2D type —, and

  • the information from the record component association list for the remaining components of the Point_2D type. (In this case, the only remaining component of the Point_2D type is Y.)

We could also specify the type of the extension aggregate. For example, in the previous assignment to P_2D, we could write Point_2D'(...) to indicate that we expect the Point_2D type for the extension aggregate.

--  Explicitly state that the type of the
--  extension aggregate is Point_2D:

P_2D := Point_2D'(P_1D with Y => 0.7);

Also, we don't have to use named association in extension aggregates. We could just use positional association instead. Therefore, we could simplify the assignment to P_2D in the previous example by just writing:

P_2D := (P_1D with 0.7);

More extension aggregates

We can use extension aggregates for descendants of the Point_2D type as well. For example, let's extend our previous code example by declaring an object of Point_3D type (called P_3D) and use extension aggregates in assignments to this object:

    
    
    
        
with Points; use Points;

procedure Show_Points is
   P_1D : Point_1D;
   P_2D : Point_2D;
   P_3D : Point_3D;
begin
   P_1D := (X => 0.5);
   Display (P_1D);

   P_2D := (P_1D with Y => 0.7);
   Display (P_2D);

   P_3D := (P_2D with Z => 0.3);
   Display (P_3D);

   P_3D := (P_1D with Y | Z => 0.1);
   Display (P_3D);
end Show_Points;

In the first assignment to P_3D in the example above, we're initializing this object with information from P_2D and specifying the value of the Z component. Then, in the next assignment to the P_3D object, we're using an aggregate with information from P_1 and specifying values for the Y and Z components. (Just as a reminder, we can write Y | Z => 0.1 to assign 0.1 to both Y and Z components.)

with others

Other versions of extension aggregates are possible as well. For example, we can combine keywords and write with others to focus on all remaining components of an extension aggregate.

    
    
    
        
with Points; use Points;

procedure Show_Points is
   P_1D : Point_1D;
   P_2D : Point_2D;
   P_3D : Point_3D;
begin
   P_1D := (X => 0.5);
   P_2D := (P_1D with Y => 0.7);

   --  Initialize P_3D with P_1D and set other
   --  components to 0.6.
   --
   P_3D := (P_1D with others => 0.6);
   Display (P_3D);

   --  Initialize P_3D with P_2D, and other
   --  components with their default value.
   --
   P_3D := (P_2D with others => <>);
   Display (P_3D);
end Show_Points;

In this example, the first assignment to P_3D has an aggregate with information from P_1D, while the remaining components — in this case, Y and Z — are just set to 0.6.

Continuing with this example, in the next assignment to P_3D, we're using information from P_2 in the extension aggregate. This covers the Point_2D part of the P_3D object — components X and Y, to be more specific. The Point_3D specific components of P_3D — component Z in this case — receive their corresponding default value. In this specific case, however, we haven't specified a default value for component Z in the declaration of the Point_3D type, so we cannot rely on any specific value being assigned to that component when using others => <>.

with null record

We can also use extension aggregates with null records. Let's focus on the P_3D_Ext object of Point_3D_Ext type. This object is declared in the Show_Points procedure of the next code example.

    
    
    
        
package Points.Extensions is

   type Point_3D_Ext is new
     Point_3D with null record;

end Points.Extensions;

    
        
with Points;            use Points;
with Points.Extensions; use Points.Extensions;

procedure Show_Points is
   P_3D     : Point_3D;
   P_3D_Ext : Point_3D_Ext;
begin
   P_3D := (X => 0.0, Y => 0.5, Z => 0.4);

   P_3D_Ext := (P_3D with null record);
   Display (P_3D_Ext);
end Show_Points;

The P_3D_Ext object is of Point_3D_Ext type, which is declared in the Points.Extensions package and derived from the Point_3D type. Note that we're not extending Point_3D_Ext with new components, but using a null record instead in the declaration. Therefore, as the Point_3D_Ext type doesn't own any new components, we just write (P_3D with null record) to initialize the P_3D_Ext object.

Extension aggregates and descendent types

In the examples above, we've been initializing objects of descendent types by using objects of ascending types in extension aggregates. We could, however, do the opposite and initialize objects of ascending types using objects of descendent type in extension aggregates. Consider this code example:

    
    
    
        
with Points; use Points;

procedure Show_Points is
   P_2D : Point_2D;
   P_3D : Point_3D;
begin
   P_3D := (X => 0.5, Y => 0.7, Z => 0.3);
   Display (P_3D);

   P_2D := (Point_1D (P_3D) with Y => 0.3);
   Display (P_2D);
end Show_Points;

Here, we're using Point_1D (P_3D) to select the Point_1D view of an object of Point_3D type. At this point, we have specified the Point_1D part of the aggregate, so we still have to specify the remaining components of the Point_2D type — the Y component, to be more specific. When we do that, we get the appropriate aggregate for the Point_2D type. In summary, by carefully selecting the appropriate view, we're able to initialize an object of ascending type (Point_2D), which contains less components, using an object of a descendent type (Point_3D), which contains more components.

Delta Aggregates

Note

This feature was introduced in Ada 2022.

Previously, we've discussed extension aggregates, which are used to assign an object Obj_From of a tagged type to an object Obj_To of a descendent type.

We may want also to assign an object Obj_From of to an object Obj_To of the same type, but change some of the components in this assignment. To do this, we use delta aggregates.

Delta Aggregates for Tagged Records

Let's reuse the Points package from a previous example:

    
    
    
        
package Points is

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

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

   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_3D) is
   begin
      Put_Line ("(X => " & P.X'Image
                & ", Y => " & P.Y'Image
                & ", Z => " & P.Z'Image & ")");
   end Display;

end Points;

    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Points is
   P1, P2, P3 : Point_3D;
begin
   P1 := (X => 0.5, Y => 0.7, Z => 0.3);
   Display (P1);

   P2 := (P1 with delta X => 1.0);
   Display (P2);

   P3 := (P1 with delta X => 0.2, Y => 0.3);
   Display (P3);
end Show_Points;

Here, we assign P1 to P2, but change the X component. Also, we assign P1 to P3, but change the X and Y components.

We can use class-wide types with delta aggregates. Consider this example:

    
    
    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Points is

   P_3D : Point_3D;

   function Reset (P_2D : Point_2D'Class)
                   return Point_2D'Class is
     ((P_2D with delta X | Y => 0.0));

begin
   P_3D := [X => 0.1, Y => 0.2, Z => 0.3];
   Display (P_3D);

   P_3D := Point_3D (Reset (P_3D));
   Display (P_3D);

end Show_Points;

In this example, the Reset function returns an object of Point_2D'Class where all components of Point_2D'Class type are zero. We call the Reset function for the P_3D object of Point_3D type, so that only the Z component remains untouched.

Note that we use the syntax X | Y in the body of the Reset function and assign the same value to both components.

For further reading...

We could have implemented Reset as a procedure — in this case, without using delta aggregates:

    
    
    
        
with Points; use Points;

procedure Show_Points is

   P_3D : Point_3D;

   procedure Reset
     (P_2D : in out Point_2D'Class) is
   begin
      Point_2D (P_2D) := (others => 0.0);
   end Reset;

begin
   P_3D := (X => 0.1, Y => 0.2, Z => 0.3);
   Display (P_3D);

   Reset (P_3D);
   Display (P_3D);

end Show_Points;

Delta Aggregates for Non-Tagged Records

The examples above use tagged types. We can also use delta aggregates with non-tagged types. Let's rewrite the Points package and convert Point_3D to a non-tagged record type.

    
    
    
        
package Points is

   type Point_3D is record
      X : Float;
      Y : Float;
      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_3D) is
   begin
      Put_Line ("(X => " & P.X'Image
                & ", Y => " & P.Y'Image
                & ", Z => " & P.Z'Image & ")");
   end Display;

end Points;

    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Points is
   P1, P2, P3 : Point_3D;
begin
   P1 := (X => 0.5, Y => 0.7, Z => 0.3);
   Display (P1);

   P2 := (P1 with delta X => 1.0);
   Display (P2);

   P3 := (P1 with delta X => 0.2, Y => 0.3);
   Display (P3);
end Show_Points;

In this example, Point_3D is a non-tagged type. Note that we haven't changed anything in the Show_Points procedure: it still works as it did with tagged types.

Delta Aggregates for Arrays

We can use delta aggregates for arrays. Let's change the declaration of Point_3D and use an array to represent a 3-dimensional point:

    
    
    
        
package Points is

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

   type Point_3D is new Float_Array (1 .. 3);

   procedure Display (P : Point_3D);

end Points;

    
        
with Ada.Text_IO; use Ada.Text_IO;

package body Points is

   procedure Display (P : Point_3D) is
   begin
      Put ("(");
      for I in P'Range loop
         Put (I'Image
              & " => "
              & P (I)'Image);
      end loop;
      Put_Line (")");
   end Display;

end Points;

    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Points is
   P1, P2, P3 : Point_3D;
begin
   P1 := [0.5, 0.7, 0.3];
   Display (P1);

   P2 := [P1 with delta 1 => 1.0];
   Display (P2);

   P3 := [P1 with delta 1 => 0.2, 2 => 0.3];
   --  Alternatively:
   --  P3 := [P1 with delta 1 .. 2 => 0.2, 0.3];

   Display (P3);
end Show_Points;

The implementation of Show_Points in this example is very similar to the version where use a record type. In this case, we:

  • assign P1 to P2, but change the first component, and

  • we assign P1 to P3, but change the first and second components.

Using slices

In the assignment to P3, we can either specify each component of the delta individually or use a slice: both forms are equivalent. Also, we can use slices to assign the same number to multiple components:

    
    
    
        
pragma Ada_2022;

with Points; use Points;

procedure Show_Points is
   P1, P3 : Point_3D;
begin
   P1 := [0.5, 0.7, 0.3];
   Display (P1);

   P3 := [P1 with delta
            P3'First + 1 .. P3'Last => 0.0];
   Display (P3);
end Show_Points;

In this example, we're assigning P1 to P3, but resetting all components of the array starting by the second one.

Multiple components

We can also assign multiple components or slices:

    
    
    
        
package Float_Arrays is

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

   procedure Display (P : Float_Array);

end Float_Arrays;

    
        
with Ada.Text_IO; use Ada.Text_IO;

package body Float_Arrays is

   procedure Display (P : Float_Array) is
   begin

      Put ("(");
      for I in P'Range loop
         Put (I'Image
              & " => "
              & P (I)'Image);
      end loop;
      Put_Line (")");

   end Display;

end Float_Arrays;

    
        
pragma Ada_2022;

with Float_Arrays; use Float_Arrays;

procedure Show_Multiple_Delta_Slices is

   P1, P2 : Float_Array (1 .. 5);

begin
   P1 := [1.0, 2.0, 3.0, 4.0, 5.0];
   Display (P1);

   P2 := [P1 with delta
            P2'First + 1 .. P2'Last - 2 => 0.0,
            P2'Last - 1  .. P2'Last => 0.2];
   Display (P2);
end Show_Multiple_Delta_Slices;

In this example, we have two arrays P1 and P2 of Float_Array type. We assign P1 to P2, but change:

  • the second to the last-but-two components to 0.0, and

  • the last-but-one and last components to 0.2.

In the Ada Reference Manual