Elaboration Control
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

Ada Reference Manual — Legal Information

# 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:
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pragma Preelaborate[(library_unit_ name)];
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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

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An elaborable construct is preelaborable unless its elaboration performs any of the following actions:
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- 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.

9/2

- The creation of an object (including a component) of a type that 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:
10.1/2

- the actual for each formal private type (or extension) declared within the formal part of the generic unit is a private type (or extension) that does not have preelaborable initialization;

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.

11/1
If a pragma Preelaborate (or pragma Pure — see below) applies to a library unit, then it is preelaborated. If a library unit is preelaborated, then its declaration, if any, and body, if any, are elaborated prior to all non-preelaborated library_items of the partition. 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. In addition to the places where Legality Rules normally apply (see 12.3), this rule applies also in the private part of an instance of a generic unit. In addition, all compilation units of a preelaborated library unit shall depend semantically only on compilation units of other preelaborated library units.
11.1/2
The following rules specify which entities have preelaborable initialization:
11.2/2

- 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, have 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.

11.4/2

- A derived type has preelaborable initialization if its parent type has preelaborable initialization and (in the case of a derived record extension) if the non-inherited components all have preelaborable initialization. However, a user-defined controlled type with an overriding Initialize 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.
11.7/2
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 private type, private extension, or protected type that is not an interface type and is without entry_declarations, 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, each component of the protected type shall have preelaborable initialization. In addition to the places where Legality Rules normally apply, 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

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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

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The form of a pragma Pure is as follows:
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pragma Pure[(library_unit_ name)];
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A pragma Pure is a library unit pragma.

#### Static Semantics

15.1/2
A pure library_item is a preelaborable library_item 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 non-visible components whose default initialization evaluates such an allocator;

15.4/2

- the elaboration of the declaration of a 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;

15.5/2

- the elaboration of the declaration of a named access-to-constant type for which the Storage_Size has been specified by an expression other than a static expression with value zero.

15.6/2
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.
17/2
A pragma Pure is used to declare that a library unit is pure. If a pragma Pure applies to a library unit, then its compilation units shall be pure, and they shall depend semantically only on compilation units of other library units that are declared pure. Furthermore, the full view of any partial view declared in the visible part of the library unit that has any available stream attributes shall support external streaming (see 13.13.2).

#### Implementation Permissions

18/2
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 are of a 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

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The form of a pragma Elaborate, Elaborate_All, or Elaborate_Body is as follows:
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pragma Elaborate(library_unit_ name{, library_unit_ name});
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pragma Elaborate_All(library_unit_ name{, library_unit_ name});
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pragma Elaborate_Body[(library_unit_ name)];
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A pragma Elaborate or Elaborate_All is only allowed within a context_clause.
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A pragma Elaborate_Body is a library unit pragma.

#### Legality Rules

25
If a pragma Elaborate_Body applies to a declaration, 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

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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. A pragma Elaborate_Body specifies that the body of the library unit is elaborated immediately after its declaration.
NOTES
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12 A preelaborated library unit is allowed to have non-preelaborable children.
28
13 A library unit that is declared pure is allowed to have impure children.

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