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10.2.1 Elaboration Control

1
This subclause defines pragmas that help control the elaboration order of library_items.

Syntax

2
The form of a pragma Preelaborate is as follows: 
3
  pragma Preelaborate[(library_unit_name)];
4
A pragma Preelaborate is a library unit pragma. 
4.1/2
The form of a pragma Preelaborable_Initialization is as follows: 
4.2/2
  pragma Preelaborable_Initialization(direct_name);

Legality Rules

5
An elaborable construct is preelaborable unless its elaboration performs any of the following actions: 
6
The execution of a statement other than a null_statement.
7
A call to a subprogram other than a static function.
8
The evaluation of a primary that is a name of an object, unless the name is a static expression, or statically denotes a discriminant of an enclosing type. 
9/3
The creation of an object (including a component) that is initialized by default, if its type does not have preelaborable initialization. Similarly, the evaluation of an extension_aggregate with an ancestor subtype_mark denoting a subtype of such a type.
10/2
 A generic body is preelaborable only if elaboration of a corresponding instance body would not perform any such actions, presuming that:
10.1/3
the actual for each discriminated formal derived type, formal private type, or formal private extension declared within the formal part of the generic unit is a type that does not have preelaborable initialization, unless pragma Preelaborable_Initialization has been applied to the formal type;
10.2/2
the actual for each formal type is nonstatic;
10.3/2
the actual for each formal object is nonstatic; and
10.4/2
the actual for each formal subprogram is a user-defined subprogram. 
11/3
 A pragma Preelaborate (or pragma Pure — see below) is used to specify that a library unit is preelaborated, namely that the Preelaborate aspect of the library unit is True; all compilation units of the library unit are preelaborated. The declaration and body of a preelaborated library unit, and all subunits that are elaborated as part of elaborating the library unit, shall be preelaborable. All compilation units of a preelaborated library unit shall depend semantically only on declared pure or preelaborated library_items. In addition to the places where Legality Rules normally apply (see 12.3), these rules also apply in the private part of an instance of a generic unit. If a library unit is preelaborated, then its declaration, if any, and body, if any, are elaborated prior to all nonpreelaborated library_items of the partition. 
11.1/2
   The following rules specify which entities have preelaborable initialization:
11.2/3
The partial view of a private type or private extension, a protected type without entry_declarations, a generic formal private type, or a generic formal derived type, has preelaborable initialization if and only if the pragma Preelaborable_Initialization has been applied to them. A protected type with entry_declarations or a task type never has preelaborable initialization.
11.3/2
A component (including a discriminant) of a record or protected type has preelaborable initialization if its declaration includes a default_expression whose execution does not perform any actions prohibited in preelaborable constructs as described above, or if its declaration does not include a default expression and its type has preelaborable initialization.
11.4/3
A derived type has preelaborable initialization if its parent type has preelaborable initialization and if the noninherited components all have preelaborable initialization. However, a controlled type with an Initialize procedure that is not a null procedure does not have preelaborable initialization.
11.5/2
A view of a type has preelaborable initialization if it is an elementary type, an array type whose component type has preelaborable initialization, a record type whose components all have preelaborable initialization, or an interface type. 
11.6/2
   A pragma Preelaborable_Initialization specifies that a type has preelaborable initialization. This pragma shall appear in the visible part of a package or generic package.
11.7/3
   If the pragma appears in the first list of basic_declarative_items of a package_specification, then the direct_name shall denote the first subtype of a composite type, and the type shall be declared immediately within the same package as the pragma. If the pragma is applied to a private type or a private extension, the full view of the type shall have preelaborable initialization. If the pragma is applied to a protected type, the protected type shall not have entries, and each component of the protected type shall have preelaborable initialization. For any other composite type, the type shall have preelaborable initialization. In addition to the places where Legality Rules normally apply (see 12.3), these rules apply also in the private part of an instance of a generic unit.
11.8/2
   If the pragma appears in a generic_formal_part, then the direct_name shall denote a generic formal private type or a generic formal derived type declared in the same generic_formal_part as the pragma. In a generic_instantiation the corresponding actual type shall have preelaborable initialization.

Implementation Advice

12
In an implementation, a type declared in a preelaborated package should have the same representation in every elaboration of a given version of the package, whether the elaborations occur in distinct executions of the same program, or in executions of distinct programs or partitions that include the given version. 

Syntax

13
The form of a pragma Pure is as follows: 
14
  pragma Pure[(library_unit_name)];
15
A pragma Pure is a library unit pragma. 

Static Semantics

15.1/3
   A pure compilation unit is a preelaborable compilation unit whose elaboration does not perform any of the following actions:
15.2/2
the elaboration of a variable declaration;
15.3/2
the evaluation of an allocator of an access-to-variable type; for the purposes of this rule, the partial view of a type is presumed to have nonvisible components whose default initialization evaluates such an allocator;
15.4/3
the elaboration of the declaration of a nonderived named access-to-variable type unless the Storage_Size of the type has been specified by a static expression with value zero or is defined by the language to be zero;
15.5/3
the elaboration of the declaration of a nonderived named access-to-constant type for which the Storage_Size has been specified by an expression other than a static expression with value zero.
15.6/3
   A generic body is pure only if elaboration of a corresponding instance body would not perform any such actions presuming any composite formal types have nonvisible components whose default initialization evaluates an allocator of an access-to-variable type.
15.7/2
   The Storage_Size for an anonymous access-to-variable type declared at library level in a library unit that is declared pure is defined to be zero.

Legality Rules

16/2
 This paragraph was deleted.
17/3
 A pragma Pure is used to specify that a library unit is declared pure, namely that the Pure aspect of the library unit is True; all compilation units of the library unit are declared pure. In addition, the limited view of any library package is declared pure. The declaration and body of a declared pure library unit, and all subunits that are elaborated as part of elaborating the library unit, shall be pure. All compilation units of a declared pure library unit shall depend semantically only on declared pure library_items. In addition to the places where Legality Rules normally apply (see 12.3), these rules also apply in the private part of an instance of a generic unit. Furthermore, the full view of any partial view declared in the visible part of a declared pure library unit that has any available stream attributes shall support external streaming (see 13.13.2). 

Erroneous Execution

17.1/4
   Execution is erroneous if some operation (other than the initialization or finalization of the object) modifies the value of a constant object declared at library-level in a pure package.

Implementation Permissions

18/3
 If a library unit is declared pure, then the implementation is permitted to omit a call on a library-level subprogram of the library unit if the results are not needed after the call. In addition, the implementation may omit a call on such a subprogram and simply reuse the results produced by an earlier call on the same subprogram, provided that none of the parameters nor any object accessible via access values from the parameters have any part that is of a type whose full type is an immutably limited type, and the addresses and values of all by-reference actual parameters, the values of all by-copy-in actual parameters, and the values of all objects accessible via access values from the parameters, are the same as they were at the earlier call. This permission applies even if the subprogram produces other side effects when called. 

Syntax

19
The form of a pragma Elaborate, Elaborate_All, or Elaborate_Body is as follows: 
20
  pragma Elaborate(library_unit_name{, library_unit_name});
21
  pragma Elaborate_All(library_unit_name{, library_unit_name});
22
  pragma Elaborate_Body[(library_unit_name)];
23
A pragma Elaborate or Elaborate_All is only allowed within a context_clause.
24
A pragma Elaborate_Body is a library unit pragma. 

Legality Rules

25/3
 If the aspect Elaborate_Body is True for a declaration (including when pragma Elaborate_Body applies), then the declaration requires a completion (a body).
25.1/2
   The library_unit_name of a pragma Elaborate or Elaborate_All shall denote a nonlimited view of a library unit. 

Static Semantics

26/3
 A pragma Elaborate specifies that the body of the named library unit is elaborated before the current library_item. A pragma Elaborate_All specifies that each library_item that is needed by the named library unit declaration is elaborated before the current library_item.
26.1/3
   A pragma Elaborate_Body sets the Elaborate_Body representation aspect of the library unit to which it applies to the value True. If the Elaborate_Body aspect of a library unit is True, the body of the library unit is elaborated immediately after its declaration.
NOTES
27
12  A preelaborated library unit is allowed to have nonpreelaborable children. 
28
13  A library unit that is declared pure is allowed to have impure children. 

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