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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 can appear anywhere that an object_declaration is allowed, 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.
NOTE   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 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

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