Allocators
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

Ada Reference Manual — Legal Information

# 4.8 Allocators

1
The evaluation of an allocator creates an object and yields an access value that designates the object.

#### Syntax

2
allocator ::=
new subtype_indication | new qualified_expression

#### Name Resolution Rules

3/1
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

4
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.
5/2
If the type of the allocator is an access-to-constant type, the allocator shall be an initialized allocator.
5.1/2
If the designated type of the type of the allocator is class-wide, the accessibility level of the type determined by the subtype_indication or qualified_expression shall not be statically deeper than that of the type of the allocator.
5.2/2
If the designated subtype of the type of the allocator has one or more unconstrained access discriminants, then the accessibility level of the anonymous access type of each access discriminant, as determined by the subtype_indication or qualified_expression of the allocator, shall not be statically deeper than that of the type of the allocator (see 3.10.2).
5.3/2
An allocator shall not be of an access type for which the Storage_Size has been specified by a static expression with value zero or is defined by the language to be zero. In addition to the places where Legality Rules normally apply (see 12.3), this rule applies also in the private part of an instance of a generic unit. This rule does not apply in the body of a generic unit or within a body declared within the declarative region of a generic unit, if the type of the allocator is a descendant of a formal access type declared within the formal part of the generic unit.

#### Static Semantics

6/2
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 subtype of the created object is the designated subtype when the designated subtype is constrained or there is a partial view of the designated type that is constrained; otherwise, the created object is constrained by its initial value (even if the designated subtype is unconstrained with defaults).

#### Dynamic Semantics

7/2
For the evaluation of an initialized allocator, the evaluation of the qualified_expression is performed first. 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.
8
For the evaluation of an uninitialized allocator, the elaboration of the subtype_indication is performed first. Then:
9/2

- If the designated type is elementary, an object of the designated subtype is created and any implicit initial value is assigned;

10/2

- 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. This object is then initialized by default (see 3.3.1) using the subtype_indication to determine its nominal subtype. 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.

10.1/2
For any allocator, if the designated type of the type of the allocator is class-wide, then a check is made that the accessibility level of the type determined by the subtype_indication, or by the tag of the value of the qualified_expression, is not deeper than that of the type of the allocator. If the designated subtype of the allocator has one or more unconstrained access discriminants, then a check is made that the accessibility level of the anonymous access type of each access discriminant is not deeper than that of the type of the allocator. Program_Error is raised if either such check fails.
10.2/2
If the object to be created by an allocator has a controlled or protected part, and the finalization of the collection of the type of the allocator (see 7.6.1) has started, Program_Error is raised.
10.3/2
If the object to be created by an allocator contains any tasks, and the master of the type of the allocator is completed, and all of the dependent tasks of the master are terminated (see 9.3), then Program_Error is raised.
11
If the created object contains any tasks, they are activated (see 9.2). Finally, an access value that designates the created object is returned.

#### Bounded (Run-Time) Errors

11.1/2
It is a bounded error if the finalization of the collection of the type (see 7.6.1) of the allocator has started. If the error is detected, Program_Error is raised. Otherwise, the allocation proceeds normally.
NOTES
12
23 Allocators cannot create objects of an abstract type. See 3.9.3.
13
24 If any part of the created object is controlled, the initialization includes calls on corresponding Initialize or Adjust procedures. See 7.6.
14
25 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.
15
26 Implementations are permitted, but not required, to provide garbage collection (see 13.11.3).

#### Examples

16
Examples of allocators:
17
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
18
new Matrix(1 .. 10, 1 .. 20) -- the bounds only are given
new Matrix'(1 .. 10 => (1 .. 20 => 0.0)) -- initialized explicitly
19
new Buffer(100) -- the discriminant only is given
new Buffer'(Size => 80, Pos => 0, Value => (1 .. 80 => 'A')) -- initialized explicitly
20
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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