13.11.2 Unchecked Storage Deallocation
1
[
Unchecked
storage deallocation of an object designated by a value of an access
type is achieved by a call to an instance of the generic procedure Unchecked_Deallocation.]
Static Semantics
2
The following language-defined
generic library procedure exists:
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generic
type Object(<>)
is limited private;
type Name
is access Object;
procedure Ada.Unchecked_Deallocation(X :
in out Name)
with Convention => Intrinsic;
pragma Preelaborate(Ada.Unchecked_Deallocation);
3.a/3
Reason: {
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The aspect Convention implies that the attribute Access is not allowed
for instances of Unchecked_Deallocation.
Legality Rules
3.1/3
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A call on an instance of Unchecked_Deallocation is illegal if the actual
access type of the instance is a 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.
3.b/3
Discussion: This rule is the same as
the rule for
allocators.
We could have left the last sentence out, as a call to Unchecked_Deallocation
cannot occur in a specification as it is a procedure call, but we left
it for consistency and to avoid future maintenance hazards.
Dynamic Semantics
4
Given an instance of
Unchecked_Deallocation declared as follows:
5
procedure Free is
new Ada.Unchecked_Deallocation(
object_subtype_name, access_to_variable_subtype_name);
6
Procedure Free has
the following effect:
7
1.
After executing Free(X), the value of X is null.
8
2.
Free(X), when X is already equal to null, has no effect.
9/3
3.
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Free(X), when X is not equal to
null first performs finalization
of the object designated by X (and any coextensions of the object —
see
3.10.2), as described in
7.6.1.
It then deallocates the storage occupied by the object designated by
X (and any coextensions). If the storage pool is a user-defined object,
then the storage is deallocated by calling Deallocate as described in
13.11. There is one exception: if the object
being freed contains tasks, the object might not be deallocated.
9.a/3
Ramification: {
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Free calls only the specified Deallocate procedure to do deallocation.
10/2
{
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After
Free(X), the object designated by X, and any subcomponents (and coextensions)
thereof, no longer exist; their storage can be reused for other purposes.
Bounded (Run-Time) Errors
11
It
is a bounded error to free a discriminated, unterminated task object.
The possible consequences are:
11.a
Reason: This is an error because the
task might refer to its discriminants, and the discriminants might be
deallocated by freeing the task object.
12
No exception is raised.
13
Program_Error
or Tasking_Error is raised at the point of the deallocation.
14
Program_Error
or Tasking_Error is raised in the task the next time it references any
of the discriminants.
14.a
Implementation Note: This last case presumes
an implementation where the task references its discriminants indirectly,
and the pointer is nulled out when the task object is deallocated.
15
In the first two cases, the storage for the discriminants
(and for any enclosing object if it is designated by an access discriminant
of the task) is not reclaimed prior to task termination.
15.a
Ramification: The storage might never
be reclaimed.
Erroneous Execution
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{
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{
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Evaluating a name that denotes
a nonexistent object, or a protected subprogram or subprogram renaming
whose associated object (if any) is nonexistent, is erroneous. The execution
of a call to an instance of Unchecked_Deallocation is erroneous if the
object was created other than by an
allocator
for an access type whose pool is Name'Storage_Pool.
16.a/3
Reason: {
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{
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The part about a protected subprogram is intended to cover the case of
an access-to-protected-subprogram where the associated object has been
deallocated. The part about a subprogram renaming is intended to cover
the case of a renaming of a prefixed view where the prefix object has
been deallocated, or the case of a renaming of an entry or protected
subprogram where the associated task or protected object has been deallocated.
16.b/3
Ramification: {
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This text does not cover the case of a name that contains a null access
value, as
null does not denote an object (rather than denoting
a nonexistent object).
Implementation Advice
17
For a standard storage pool, Free should actually
reclaim the storage.
17.a.1/2
Implementation Advice: For a standard
storage pool, an instance of Unchecked_Deallocation should actually reclaim
the storage.
17.a/2
Ramification: {
AI95-00114-01}
This is not a testable property, since we do not know how much storage
is used by a given pool element, nor whether fragmentation can occur.
17.1/3
{
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A call on an instance of Unchecked_Deallocation with a nonnull access
value should raise Program_Error if the actual access type of the instance
is a type for which the Storage_Size has been specified to be zero or
is defined by the language to be zero.
17.a.1/3
Implementation Advice: A call on an instance
of Unchecked_Deallocation with a nonnull access value should raise Program_Error
if the actual access type of the instance is a type for which the Storage_Size
has been specified to be zero or is defined by the language to be zero.
17.b
Discussion: If the call is not illegal
(as in a generic body), we recommend that it raise Program_Error. Since
the execution of this call is erroneous (any allocator from the pool
will have raised Storage_Error, so the nonnull access value must have
been allocated from a different pool or be a stack-allocated object),
we can't require any behavior — anything at all would be a legitimate
implementation.
18
30 The rules here that refer to Free apply
to any instance of Unchecked_Deallocation.
19
31 Unchecked_Deallocation cannot be instantiated
for an access-to-constant type. This is implied by the rules of
12.5.4.
Wording Changes from Ada 95
19.a/2
{
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The rules for coextensions are clarified (mainly by adding that term).
In theory, this reflects no change from Ada 95 (coextensions existed
in Ada 95, they just didn't have a name).
Wording Changes from Ada 2005
19.b/3
{
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Correction: Added a rule that using an access-to-protected-subprogram
is erroneous if the associated object no longer exists. It is hard to
imagine an alternative meaning here, and this has no effect on correct
programs.
19.c/3
{
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Correction: Moved the requirements on an implementation-generated
call to Deallocate to
13.11, in order to
put all of the rules associated with implementation-generated calls to
Allocate and Deallocate together.
19.d/3
{
AI05-0157-1}
Correction: Added wording so that calling an instance of Unchecked_Deallocation
is treated similarly to
allocators
for access types where
allocators
would be banned.
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