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4.8 Allocators

1
[The evaluation of an allocator creates an object and yields an access value that designates the object. {new: See allocator} {malloc: See allocator} {heap management: See also allocator} ]

Syntax

2
allocator ::= 
   new subtype_indication | new qualified_expression

Name Resolution Rules

3/1
{8652/0010} {AI95-00127-01} {expected type (allocator) [partial]} The expected type for an allocator shall be a single access-to-object type with whose 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.
3.a
Discussion: See 8.6, “The Context of Overload Resolution” for the meaning of “shall be a single ... type whose ...”
3.a.1/1
Ramification: {8652/0010} {AI95-00127-01} An allocator is allowed as a controlling parameter of a dispatching call (see 3.9.2).

Legality Rules

4
{initialized allocator} An initialized allocator is an allocator with a qualified_expression. {uninitialized allocator} 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. {constructor: See initialized allocator}
4.a
Ramification: For example, ... new S'Class ... (with no initialization expression) is illegal, but ... new S'Class'(X) ... is legal, and takes its tag and constraints from the initial value X. (Note that the former case cannot have a constraint.)
5/2
{AI95-00287-01} 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. 
5.a/2
This paragraph was deleted.Ramification: {AI95-00287-01} For an access-to-constant type whose designated type is limited, allocators are illegal. The Access attribute is legal for such a type, however.
5.1/2
  {AI95-00344-01} 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.b/2
Reason: This prevents the allocated object from outliving its type. 
5.2/2
  {AI95-00416-01} 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.c/2
Reason: This prevents the allocated object from outliving its discriminants. 
5.3/2
  {AI95-00366-01} 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. {generic contract issue [partial]} 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.
5.d/2
Reason: An allocator for an access type that has Storage_Size specified to be zero is required to raise Storage_Error anyway. It's better to detect the error at compile-time, as the allocator might be executed infrequently. This also simplifies the rules for Pure units, where we do not want to allow any allocators for library-level access types, as they would represent state.
5.e/2
The last sentence covers the case of children of generics, and formal access types of formal packages of the generic unit. 

Static Semantics

6/2
{AI95-00363-01} 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 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)]. {constrained by its initial value [partial]}
6.a
Discussion: See AI83-00331. 
6.b/2
Reason: {AI95-00363-01} All objects created by an allocator allocator are aliased, and most all aliased composite objects need to be constrained so that access subtypes work reasonably. Problematic access subtypes are prohibited for types with a constrained partial view. 
6.c/2
Discussion: {AI95-00363-01} If there is a constrained partial view of the type, this allows the objects to be unconstrained. This eliminates privacy breaking (we don't want the objects to act differently simply because they're allocated). Such a created object is effectively constrained by its initial value if the access type is an access-to-constant type, or the designated type is limited (in all views), but we don't need to state that here. It is implicit in other rules. Note, however, that a value of an access-to-constant type can designate a variable object via 'Access or conversion, and the variable object might be assigned by some other access path, and that assignment might alter the discriminants.

Dynamic Semantics

7/2
{AI95-00373-01} {evaluation (allocator) [partial]} For the evaluation of an initialized allocator allocator, the elaboration of the subtype_indication or the evaluation of the qualified_expression is performed first. {evaluation (initialized allocator) [partial]} {assignment operation (during evaluation of an initialized allocator)} An 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. {implicit subtype conversion (initialization expression of allocator) [partial]}
7.a
Ramification: The conversion might raise Constraint_Error. 
8
{evaluation (uninitialized allocator) [partial]} For the evaluation of an uninitialized allocator, the elaboration of the subtype_indication is performed first. Then
9/2
10/2
10.a
Discussion: AI83-00150. 
10.1/2
   {AI95-00344-01} {AI95-00416-01} 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.{Accessibility_Check [partial]} {check, language-defined (Accessibility_Check)} {Program_Error (raised by failure of run-time check)}
10.b/2
Reason: {AI95-00344-01} The accessibility check on class-wide types prevents the allocated object from outliving its type. We need the run-time check in instance bodies, or when the type of the qualified_expression is class-wide (other cases are statically detected).
10.c/2
{AI95-00416-01} The accessibility check on access discriminants prevents the allocated object from outliving its discriminants.
10.2/2
   {AI95-00280-01} 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.{Allocation_Check [partial]} {check, language-defined (Allocation_Check)} {Program_Error (raised by failure of run-time check)}
10.d/2
Reason: If the object has a controlled or protected part, its finalization is likely to be non-trivial. If the allocation was allowed, we could not know whether the finalization would actually be performed. That would be dangerous to otherwise safe abstractions, so we mandate a check here. On the other hand, if the finalization of the object will be trivial, we do not require (but allow) the check, as no real harm could come from late allocation.
10.e/2
Discussion: This check can only fail if an allocator is evaluated in code reached from a Finalize routine for a type declared in the same master. That's highly unlikely; Finalize routines are much more likely to be deallocating objects than allocating them. 
10.3/2
   {AI95-00280-01} 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.{Allocation_Check [partial]} {check, language-defined (Allocation_Check)} {Program_Error (raised by failure of run-time check)}
10.f/2
Reason: A task created after waiting for tasks has finished could depend on freed data structures, and certainly would never be awaited. 
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
   {AI95-00280-01} {bounded error (cause) [partial]} 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. 
11.a/2
Discussion: This check is required in some cases; see above. 
NOTES
12
24  Allocators cannot create objects of an abstract type. See 3.9.3.
13
25  If any part of the created object is controlled, the initialization includes calls on corresponding Initialize or Adjust procedures. See 7.6.
14
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). {Storage_Error (raised by failure of run-time check)} 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
27  Implementations are permitted, but not required, to provide garbage collection (see 13.11.3).
15.a
Ramification: Note that in an allocator, the exception Constraint_Error can be raised by the evaluation of the qualified_expression, by the elaboration of the subtype_indication, or by the initialization. 
15.b
Discussion: By default, the implementation provides the storage pool. The user may exercise more control over storage management by associating a user-defined pool with an access type. 

Examples

16
Examples of allocators:
17
new Cell'(0, nullnull)                          -- 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

Incompatibilities With Ada 83

20.a/1
{incompatibilities with Ada 83} The subtype_indication of an uninitialized allocator may not have an explicit constraint if the designated type is an access type. In Ada 83, this was permitted even though the constraint had no e affect on the subtype of the created object. 

Extensions to Ada 83

20.b
{extensions to Ada 83} Allocators creating objects of type T are now overloaded on access types designating T'Class and all class-wide types that cover T.
20.c
Implicit array subtype conversion (sliding) is now performed as part of an initialized allocator. 

Wording Changes from Ada 83

20.d
We have used a new organization, inspired by the ACID document, that makes it clearer what is the subtype of the created object, and what subtype conversions take place.
20.e
Discussion of storage management issues, such as garbage collection and the raising of Storage_Error, has been moved to 13.11, “Storage Management”. 

Inconsistencies With Ada 95

20.f/2
{AI95-00363-01} {inconsistencies with Ada 95} If the designated type has a constrained partial view, the allocated object can be unconstrained. This might cause the object to take up a different amount of memory, and might cause the operations to work where they previously would have raised Constraint_Error. It's unlikely that the latter would actually matter in a real program (Constraint_Error usually indicates a bug that would be fixed, not left in a program.) The former might cause Storage_Error to be raised at a different time than in an Ada 95 program.

Incompatibilities With Ada 95

20.g/2
{AI95-00366-01} {incompatibilities with Ada 95} An allocator for an access type that has Storage_Size specified to be zero is now illegal. Ada 95 allowed the allocator, but it had to raise Storage_Error if executed. The primary impact of this change should be to detect bugs. 

Extensions to Ada 95

20.h/2
{8652/0010} {AI95-00127-01} {extensions to Ada 95} Corrigendum: An allocator can be a controlling parameter of a dispatching call. This was an oversight in Ada 95.
20.i/2
{AI95-00287-01} Initialized allocators are allowed when the designated type is limited. 

Wording Changes from Ada 95

20.j/2
{8652/0002} {AI95-00171-01} Corrigendum: Clarified the elaboration of per-object constraints for an uninitialized allocator.
20.k/2
{AI95-00280-01} Program_Error is now raised if the allocator occurs after the finalization of the collection or the waiting for tasks. This is not listed as an incompatibility as the Ada 95 behavior was unspecified, and Ada 95 implementations tend to generate programs that crash in this case.
20.l/2
{AI95-00344-01} Added accessibility checks to class-wide allocators. These checks could not fail in Ada 95 (as all of the designated types had to be declared at the same level, so the access type would necessarily have been at the same level or more nested than the type of allocated object).
20.m/2
{AI95-00373-01} Revised the description of evaluation of uninitialized allocators to use “initialized by default” so that the ordering requirements are the same for all kinds of objects that are default-initialized.
20.n/2
{AI95-00416-01} Added accessibility checks to access discriminants of allocators. These checks could not fail in Ada 95 as the discriminants always have the accessibility of the object. 

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