Ada Reference Manual (Ada 2022 Draft 34) — Legal Information

4.5.9 Declare Expressions

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Declare expressions provide a way to declare local constants and object renamings in an expression context.

Syntax

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declare_expression ::=
declare {declare_item}
begin body_expression

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declare_item ::= object_declaration | object_renaming_declaration

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Wherever the Syntax Rules allow an expression, a declare_expression may be used in place of the expression, so long as it is immediately surrounded by parentheses.

Legality Rules

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A declare_item that is an object_declaration shall declare a constant of a nonlimited type.

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A declare_item that is an object_renaming_declaration (see 8.5.1) shall not rename an object of a limited type if any operative constituent of the object_name is a value conversion or is newly constructed (see 4.4).

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The following are not allowed within a declare_expression: a declaration containing the reserved word aliased; the attribute_designator Access or Unchecked_Access; or an anonymous access type.

Name Resolution Rules

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If a declare_expression is expected to be of a type T, then the body_expression is expected to be of type T. Similarly, if a declare_expression is expected to be of some class of types, then the body_expression is subject to the same expectation. If a declare_expression shall resolve to be of a type T, then the body_expression shall resolve to be of type T.

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The type of a declare_expression is the type of the body_expression.

Dynamic Semantics

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For the evaluation of a declare_expression, the declare_items are elaborated in order, and then the body_expression is evaluated. The value of the declare_expression is that of the body_expression.

Examples

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Example of use of a declare expression as a replacement postcondition for Ada.Containers.Vectors."&" (see A.18.2):

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with Post =>
   (declare
      Result renames Vectors."&"'Result;
      Length : constant Count_Type := Left.Length + Right.Length;
    begin
      Result.Length = Length and then
      not Tampering_With_Elements_Prohibited (Result) and then
      not Tampering_With_Cursors_Prohibited (Result) and then
      Result.Capacity >= Length)

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