In contrast to the entities declared in the visible part of a package specification, the entities declared in the package body are only visible 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 (a package in the usual sense), in which the logical operations available to the users are clearly isolated from the internal entities.
For the elaboration of a package body, its declarative part is first elaborated, and its sequence of statements (if any) is then executed. The optional exception handlers at the end of a package body service exceptions raised during the execution of the sequence of statements of the package body.
Notes:
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 an "own" variable of Algol 60.
The elaboration of the body of a subprogram 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 3.9).
Example of a package:
package RATIONAL_NUMBERS is type RATIONAL is record NUMERATOR : INTEGER; DENOMINATOR : POSITIVE; end record; function EQUAL(X,Y : RATIONAL) return BOOLEAN; function "/" (X,Y : INTEGER) return RATIONAL; -- to construct a rational number function "+" (X,Y : RATIONAL) return RATIONAL; function "-" (X,Y : RATIONAL) return RATIONAL; function "*" (X,Y : RATIONAL) return RATIONAL; function "/" (X,Y : RATIONAL) return RATIONAL; end; package body RATIONAL_NUMBERS is procedure SAME_DENOMINATOR (X,Y : in out RATIONAL) is begin -- reduces X and Y to the same denominator: ... end; function EQUAL(X,Y : RATIONAL) return BOOLEAN is U,V : RATIONAL; begin U := X; V := Y; SAME_DENOMINATOR (U,V); return U.NUMERATOR = V.NUMERATOR; end EQUAL; 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;
References: declaration, declarative part, elaboration, and 3.9, exception, exception handler, name, package specification, program unit, program_error exception, sequence of statements, subprogram, variable, visible part.
Rationale references: 9.3 Technical Issues, 10.5.3 (Ch 10 Separate Compilation) Treatment of Package Bodies
Style Guide references: 2.1.7 Pagination, 3.3.4 Program Unit Body Header, 4.1.1 Separate Compilation Capabilities, 4.2.1 Minimization of Interfaces, 4.3.1 Using Exceptions to Help Define an Abstraction, 5.1.4 Naming End Statements, 5.9.1 Unchecked Conversion
Address any questions or comments to adainfo@sw-eng.falls-church.va.us.