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.
A deferred
constant declaration requires a completion, which shall be a full constant
declaration (called the
full declaration of the deferred constant),
or a
pragma
Import (see
Annex B).
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
that defines a constrained subtype, then the subtype defined by the
subtype_indication
in the full declaration shall match it 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 that is completed
by a
pragma
Import 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
13 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 7.3.1CPU_Identifier :
constant String(1..8);
pragma Import(Assembler, CPU_Identifier, Link_Name => "CPU_ID");
--
see B.1
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