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Correction: update the stored-empty container aggregate example #1396

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gusthoff merged 3 commits into from
Oct 30, 2026
Merged

Correction: update the stored-empty container aggregate example #1396

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a93c493
Correction: use a function for an empty container aggregate
gusthoff Oct 25, 2026
be6e715
Correction: run the stored-empty container aggregate example
gusthoff Oct 25, 2026
1a0a6f4
Correction: wrap the stored-empty example output
gusthoff Oct 30, 2026
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30 changes: 24 additions & 6 deletions content/courses/advanced-ada/parts/resource_management/containers.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -214,20 +214,21 @@ can confirm this by running this example and seeing the
:ada:`Calling Empty_Func` message, which we included in the body of the
:ada:`Empty_Func` function.)

We can also use a constant for the :ada:`Empty` element instead of a function:
An :ada:`Empty` function can also return a stored constant:

.. code:: ada no_button gnat=14.2.0-1 project=Courses.Advanced_Ada.Aggregates.Container_Aggregate_Empty_Const switches=Compiler(-gnat2022);
:class: ada-compile
.. code:: ada run_button project=Courses.Advanced_Ada.Aggregates.Container_Aggregate_Empty_Const switches=Compiler(-gnat2022);

package Custom_Container_Aggregates is

type T is private
with Aggregate =>
(Empty => Empty_Const,
(Empty => Empty_Func,
Add_Named => Add_Named_Proc);

Empty_Const : constant T;

function Empty_Func return T;

procedure Add_Named_Proc
(Cont : in out T;
Key : String;
Expand All @@ -241,10 +242,27 @@ We can also use a constant for the :ada:`Empty` element instead of a function:

Empty_Const : constant T := (Cnt => 0);

function Empty_Func return T is (Empty_Const);

end Custom_Container_Aggregates;

Here, we simply assign :ada:`Empty_Const` when an actual :ada:`Empty` is
needed.
with Ada.Text_IO; use Ada.Text_IO;

with Custom_Container_Aggregates;
use Custom_Container_Aggregates;

procedure Show_Container_Aggregate_Empty_Const is
A : constant T := [];
begin
Put_Line ("A = Empty_Const: "
& Boolean'Image (A = Empty_Const));
end Show_Container_Aggregate_Empty_Const;

Here, :ada:`Empty_Const` stores the empty value, and :ada:`Empty_Func` is an
expression function that returns it. We complete the function in the private
part, after the full declaration of the constant, so no package body is needed.
Initializing :ada:`A` with :ada:`[]` calls :ada:`Empty_Func`; the program prints
``A = Empty_Const: TRUE``.

In addition to this, we can specify a signed integer parameter |mdash| which
indicates the number of components |mdash| for the :ada:`Empty` function:
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