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7.2 Package Bodies

In contrast to the entities declared in the visible part of a package, the entities declared in the package_body are visible only within the package_body itself. As a consequence, a package with a package_body can be used for the construction of a group of related subprograms in which the logical operations available to clients are clearly isolated from the internal entities.

Syntax

   
       package_body ::=
           package body defining_program_unit_name is
              declarative_part
          [begin
               handled_sequence_of_statements]
           end [[parent_unit_name.]identifier];

If an identifier or parent_unit_name.identifier appears at the end of a package_body, then this sequence of lexical elements shall repeat the defining_program_unit_name.

Legality Rules

A package_body shall be the completion of a previous package_declaration or generic_package_declaration. A library package_declaration or library generic_package_declaration shall not have a body unless it requires a body; pragma Elaborate_Body can be used to require a library_unit_declaration to have a body (see 10.2.1) if it would not otherwise require one.

Static Semantics

In any package_body without statements there is an implicit null_statement. For any package_declaration without an explicit completion, there is an implicit package_body containing a single null_statement. For a noninstance, nonlibrary package, this body occurs at the end of the declarative_part of the innermost enclosing program unit or block_statement; if there are several such packages, the order of the implicit package_bodies is unspecified. (For an instance, the implicit package_body occurs at the place of the instantiation (see 12.3). For a library package, the place is partially determined by the elaboration dependences (see Section 10).)

Dynamic Semantics

For the elaboration of a nongeneric package_body, its declarative_part is first elaborated, and its handled_sequence_of_statements is then executed.

NOTES

(3) A variable declared in the body of a package is only visible within this body and, consequently, its value can only be changed within the package_body. In the absence of local tasks, the value of such a variable remains unchanged between calls issued from outside the package to subprograms declared in the visible part. The properties of such a variable are similar to those of a ``static'' variable of C.

(4) The elaboration of the body of a subprogram explicitly declared in the visible part of a package is caused by the elaboration of the body of the package. Hence a call of such a subprogram by an outside program unit raises the exception Program_Error if the call takes place before the elaboration of the package_body (see 3.11).

Examples

Example of a package body (see 7.1):

    
       package body Rational_Numbers is

          procedure Same_Denominator (X,Y : in out Rational) is
          begin
             --  reduces X and Y to the same denominator:
             ...
          end Same_Denominator;

          function "="(X,Y : Rational) return Boolean is
             U : Rational := X;
             V : Rational := Y;
          begin
             Same_Denominator (U,V);
             return U.Numerator = V.Numerator;
          end "=";

          function "/" (X,Y : Integer) return Rational is
          begin
             if Y > 0 then
                return (Numerator => X,  Denominator => Y);
             else
                return (Numerator => -X, Denominator => -Y);
             end if;
          end "/";

          function "+" (X,Y : Rational) return Rational is ...  end "+";
          function "-" (X,Y : Rational) return Rational is ...  end "-";
          function "*" (X,Y : Rational) return Rational is ...  end "*";
          function "/" (X,Y : Rational) return Rational is ...  end "/";

       end Rational_Numbers;

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