Subunits of Compilation Units

[Subunits are like child units, with these (important) differences: subunits support the separate compilation of bodies only (not declarations); the parent contains a body_stub to indicate the existence and place of each of its subunits; declarations appearing in the parent's body can be visible within the subunits.]

Syntax

Discussion: Although this syntax allows a parent_unit_name, that is disallowed by 10.1.1, “Compilation Units - Library Units”.

Legality Rules

{AI95-00243-01} {parent body (of a subunit)} The parent body of a subunit is the body of the program unit denoted by its parent_unit_name. {subunit} The term subunit is used to refer to a subunit and also to the proper_body of a subunit. The subunits of a program unit include any subunit that names that program unit as its parent, as well as any subunit that names such a subunit as its parent (recursively).{subunit (of a program unit)}

Reason: {AI95-00243-01} We want any rule that applies to a subunit to apply to a subunit of a subunit as well.

The parent body of a subunit shall be present in the current environment, and shall contain a corresponding body_stub with the same defining_identifier as the subunit.

Discussion: This can't be a Name Resolution Rule, because a subunit is not a complete context.

A package_body_stub shall be the completion of a package_declaration or generic_package_declaration; a task_body_stub shall be the completion of a task declaration; a protected_body_stub shall be the completion of a protected declaration.

In contrast, a subprogram_body_stub need not be the completion of a previous declaration, [in which case the _stub declares the subprogram]. If the _stub is a completion, it shall be the completion of a subprogram_declaration or generic_subprogram_declaration. The profile of a subprogram_body_stub that completes a declaration shall conform fully to that of the declaration. {full conformance (required)}

Discussion: The part about subprogram_body_stubs echoes the corresponding rule for subprogram_bodies in 6.3, “Subprogram Bodies”.

A subunit that corresponds to a body_stub shall be of the same kind (package_, subprogram_, task_, or protected_) as the body_stub. The profile of a subprogram_body subunit shall be fully conformant to that of the corresponding body_stub. {full conformance (required)}

A body_stub shall appear immediately within the declarative_part of a compilation unit body. This rule does not apply within an instance of a generic unit.

Discussion: {methodological restriction} This is a methodological restriction; that is, it is not necessary for the semantics of the language to make sense.

The defining_identifiers of all body_stubs that appear immediately within a particular declarative_part shall be distinct.

Post-Compilation Rules

4 The rules in 10.1.4, “The Compilation Process” say that a body_stub is equivalent to the corresponding proper_body. This implies:

Visibility within a subunit is the visibility that would be obtained at the place of the corresponding body_stub (within the parent body) if the context_clause of the subunit were appended to that of the parent body.

Ramification: Recursively. Note that this transformation might make the parent illegal; hence it is not a true equivalence, but applies only to visibility within the subunit.

Ramification: The elaboration of a subunit is part of its parent body's elaboration, whereas the elaboration of a child unit is not part of its parent declaration's elaboration.

Ramification: A library_item that is mentioned in a with_clause of a subunit can be hidden (from direct visibility) by a declaration (with the same identifier) given in the subunit. Moreover, such a library_item can even be hidden by a declaration given within the parent body since a library unit is declared in its parent's declarative region; this however does not affect the interpretation of the with_clauses themselves, since only library_items are visible or directly visible in with_clauses.

The body of a protected operation cannot be a subunit. This follows from the syntax rules. The body of a protected unit can be a subunit.

Examples

with Ada.Text_IO;
package body Parent is
    Variable : String := "Hello, there.";
    procedure Inner is
    begin
        Ada.Text_IO.Put_Line(Variable);
    end Inner;
end Parent;

The body of procedure Inner may be turned into a subunit by rewriting the package body as follows (with the declaration of Parent remaining the same):

package body Parent is
Variable : String := "Hello, there.";
procedure Inner is separate;
end Parent;

with Ada.Text_IO;
separate(Parent)
procedure Inner is
begin
Ada.Text_IO.Put_Line(Variable);
end Inner;

Extensions to Ada 83

{extensions to Ada 83} Subunits of the same ancestor library unit are no longer restricted to have distinct identifiers. Instead, we require only that the full expanded names be distinct.

Extensions to Ada 95

{AI95-00218-03} {extensions to Ada 95} An overriding_indicator (see 8.3.1) is allowed on a subprogram stub.