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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.
]
Syntax
2
allocator
::=
new subtype_indication |
new qualified_expression
Name Resolution Rules
3/1
{
8652/0010}
The expected type for an
allocator
shall be a single access-to-object type
withwhose 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
3.a.1/1
Ramification: {8652/0010}
An allocator is allowed as a controlling parameter of a dispatching
call (see 3.9.2).
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.
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
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
Ramification: For an access-to-constant
type whose designated type is limited, allocators
are illegal. The Access attribute is legal for such a type, however.
Static Semantics
6
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)].
6.a
Discussion: See AI83-00331.
6.b
Reason: All objects created
by an allocator are aliased, and all aliased composite objects need to
be constrained so that access subtypes work reasonably.
Dynamic Semantics
7
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.
7.a
Ramification: The conversion
might raise Constraint_Error.
8
For the evaluation of an uninitialized
allocator:
9
- If the designated type is elementary,
an object of the designated subtype is created and any implicit initial
value is assigned;
10/1
- {8652/0002}
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.
10.a
Discussion: AI83-00150.
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.
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). 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, 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
Incompatibilities With Ada 83
20.a/1
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
eaffect on the subtype of the created object.
Extensions to Ada 83
20.b
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''.
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