10.1.3 Subunits of Compilation Units

1. 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
   

2. body_stub ::=
        subprogram_body_stub
      | package_body_stub
      | task_body_stub
      | protected_body_stub
   

3. subprogram_body_stub ::= subprogram_specification is separate;
   

4. package_body_stub ::= package body defining_identifier is separate;
   

5. task_body_stub ::= task body defining_identifier is separate;
   

6. protected_body_stub ::=
      protected body defining_identifier is separate;
   

7. subunit ::= separate (parent_unit_name) proper_body
   

   Legality Rules
   
8. The parent body of a subunit is the body of the program unit denoted by its parent_unit_name. The term subunit is used to refer to a subunit and also to the proper_body of a subunit.
9. 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.
10. 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.
11. 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.
12. 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.
13. 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.
14. The defining_identifiers of all body_stubs that appear immediately within a particular declarative_part shall be distinct.
    Post-Compilation Rules
    
15. For each body_stub, there shall be a subunit containing the corresponding proper_body.
    

    NOTES
    

16. (4) The rules in See section 10.1.4 The Compilation Process, say that a body_stub is equivalent to the corresponding proper_body. This implies:
    1. 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.
    2. The effect of the elaboration of a body_stub is to elaborate the subunit.

1. The package Parent is first written without subunits:

2. package Parent is
       procedure Inner;
   end Parent;
   

3. 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;
   

4. 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):

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

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

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