4.8 Allocators

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The evaluation of an allocator creates an object and yields an access value that designates the object.


allocator ::=
   new subtype_indication | new qualified_expression

Name Resolution Rules

The expected type for an allocator shall be a single access-to-object type with designated type D such that either D covers the type determined by the subtype_mark of the subtype_indication or qualified_expression, or the expected type is anonymous and the determined type is D'Class.

Legality Rules

An initialized allocator is an allocator with a qualified_expression. An uninitialized allocator is one with a subtype_indication. In the subtype_indication of an uninitialized allocator, a constraint is permitted only if the subtype_mark denotes an unconstrained composite subtype; if there is no constraint, then the subtype_mark shall denote a definite subtype.

If the type of the allocator is an access-to-constant type, the allocator shall be an initialized allocator. If the designated type is limited, the allocator shall be an uninitialized allocator.

Static Semantics

If the designated type of the type of the allocator is elementary, then the subtype of the created object is the designated subtype. If the designated type is composite, then the created object is always constrained; if the designated subtype is constrained, then it provides the constraint of the created object; otherwise, the object is constrained by its initial value (even if the designated subtype is unconstrained with defaults).

Dynamic Semantics

For the evaluation of an allocator, the elaboration of the subtype_indication or the evaluation of the qualified_expression is performed first. For the evaluation of an initialized allocator, an object of the designated type is created and the value of the qualified_expression is converted to the designated subtype and assigned to the object.

For the evaluation of an uninitialized allocator:

  • If the designated type is elementary, an object of the designated subtype is created and any implicit initial value is assigned;
  • If the designated type is composite, an object of the designated type is created with tag, if any, determined by the subtype_mark of the subtype_indication; any per-object constraints on subcomponents are elaborated (see 3.8) and any implicit initial values for the subcomponents of the object are obtained as determined by the subtype_indication and assigned to the corresponding subcomponents. A check is made that the value of the object belongs to the designated subtype. Constraint_Error is raised if this check fails. This check and the initialization of the object are performed in an arbitrary order.

If the created object contains any tasks, they are activated (see 9.2). Finally, an access value that designates the created object is returned.


24  Allocators cannot create objects of an abstract type. See 3.9.3.

25  If any part of the created object is controlled, the initialization includes calls on corresponding Initialize or Adjust procedures. See 7.6.

26  As explained in 13.11, Storage Management, the storage for an object allocated by an allocator comes from a storage pool (possibly user defined). The exception Storage_Error is raised by an allocator if there is not enough storage. Instances of Unchecked_Deallocation may be used to explicitly reclaim storage.

27  Implementations are permitted, but not required, to provide garbage collection (see 13.11.3).


Examples of allocators:

new Cell'(0, null, null)                          -- initialized explicitly, see 3.10.1
new Cell'(Value => 0, Succ => null, Pred => null) -- initialized explicitly
new Cell                                          -- not initialized

new Matrix(1 .. 10, 1 .. 20)                      -- the bounds only are given
new Matrix'(1 .. 10 => (1 .. 20 => 0.0))          -- initialized explicitly

new Buffer(100)                                   -- the discriminant only is given
new Buffer'(Size => 80, Pos => 0, Value => (1 .. 80 => 'A')) -- initialized explicitly

Expr_Ptr'(new Literal)                  -- allocator for access-to-class-wide type, see 3.9.1 
Expr_Ptr'(new Literal'(Expression with 3.5))      -- initialized explicitly

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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual