Ada 2012 Reference Manual

7.4 Deferred Constants

Deferred constant declarations may be used to declare constants in the visible part of a package, but with the value of the constant given in the private part. They may also be used to declare constants imported from other languages (see Annex B).

Legality Rules

A deferred constant declaration is an object_declaration with the reserved word constant but no initialization expression. The constant declared by a deferred constant declaration is called a deferred constant. Unless the Import aspect (see B.1) is True for a deferred constant declaration, the deferred constant declaration requires a completion, which shall be a full constant declaration (called the full declaration of the deferred constant).

A deferred constant declaration that is completed by a full constant declaration shall occur immediately within the visible part of a package_specification. For this case, the following additional rules apply to the corresponding full declaration:

The full declaration shall occur immediately within the private part of the same package;

The deferred and full constants shall have the same type, or shall have statically matching anonymous access subtypes;

If the deferred constant declaration includes a subtype_indication S that defines a constrained subtype, then the constraint defined by the subtype_indication in the full declaration shall match the constraint defined by S statically. On the other hand, if the subtype of the deferred constant is unconstrained, then the full declaration is still allowed to impose a constraint. The constant itself will be constrained, like all constants;

If the deferred constant declaration includes the reserved word aliased, then the full declaration shall also;

If the subtype of the deferred constant declaration excludes null, the subtype of the full declaration shall also exclude null.

A deferred constant declaration for which the Import aspect is True need not appear in the visible part of a package_specification, and has no full constant declaration.

The completion of a deferred constant declaration shall occur before the constant is frozen (see 13.14).

Dynamic Semantics

The elaboration of a deferred constant declaration elaborates the subtype_indication, access_definition, or (only allowed in the case of an imported constant) the array_type_definition.

NOTES

14 The full constant declaration for a deferred constant that is of a given private type or private extension is not allowed before the corresponding full_type_declaration. This is a consequence of the freezing rules for types (see 13.14).

Examples

Examples of deferred constant declarations:

Null_Key : constant Key; -- see 7.3.1

CPU_Identifier : constant String(1..8)
with Import => True, Convention => Assembler, Link_Name => "CPU_ID";
-- see B.1