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13.11.2 Unchecked Storage Deallocation

[ 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

The following language-defined generic library procedure exists: 
{AI05-0229-1} 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);
Reason: {AI05-0229-1} The aspect Convention implies that the attribute Access is not allowed for instances of Unchecked_Deallocation. 

Legality Rules

  {AI05-0157-1} 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.
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

Given an instance of Unchecked_Deallocation declared as follows: 
procedure Free is
    new Ada.Unchecked_Deallocation(
Procedure Free has the following effect: 
After executing Free(X), the value of X is null.
Free(X), when X is already equal to null, has no effect.
{AI95-00416-01} {AI05-0107-1} 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. 
Ramification: {AI05-0107-1} Free calls only the specified Deallocate procedure to do deallocation.
 {AI95-00416-01} {AI12-0148-1} After the finalization step of 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

It is a bounded error to free a discriminated, unterminated task object. The possible consequences are: 
Reason: This is an error because the task might refer to its discriminants, and the discriminants might be deallocated by freeing the task object. 
No exception is raised.
Program_Error or Tasking_Error is raised at the point of the deallocation.
Program_Error or Tasking_Error is raised in the task the next time it references any of the discriminants. 
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. 
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. 
Ramification: The storage might never be reclaimed. 
   {AI12-0148-1} An access value that designates a nonexistent object is called a dangling reference.
Discussion: These can result from use of Unchecked_Deallocation, Unchecked_Deallocate_Subpool, and attribute Unchecked_Access. Bad results from Unchecked_Conversion and from stream-oriented attributes are abnormal by 13.9.1, which is stronger and thus takes precedence. 
   {AI12-0148-1} [If a dangling reference is dereferenced (implicitly or explicitly), execution is erroneous (see below).] If there is no explicit or implicit dereference, then it is a bounded error to evaluate an expression whose result is a dangling reference. If the error is detected, either Constraint_Error or Program_Error is raised. Otherwise, execution proceeds normally, but with the possibility that the access value designates some other existing object.
Reason: If a dangling reference is compared with another access value, a result of either True or False is allowed. We need to allow this so that simple implementations of access values (for instance, as a bare address) can work if the memory in question is reused. (The formal definition of access equality is that it returns True if both access values designate the same object; that can never be True if one of the values is a dangling reference, and the other is not, but both values could refer to the same memory.) Membership tests that do not involve an implicit dereference generally do not depend on the access value at all.
We allow Constraint_Error to be raised here so that dangling reference and null pointer checks can be combined into a single check. If different exceptions are required, then the checks have to be made separately - but there's little semantic difference (neither designate a usable object). 
Ramification: If a dangling reference is assigned into an object, including being passed to a formal parameter, that object also contains a dangling reference afterwards. 
Discussion: For equality and membership operations on composite types, this applies to any parts that are access types, as these operations are created based on the operations of the components (which triggers the bounded error). For other operations on composite types, the bounded error is not triggered. For instance, an assignment of a composite object with a subcomponent that is a dangling reference has to work normally; no exception can be raised, but the target object will have a subcomponent that is a dangling references, and a (direct) use of that subcomponent is again a bounded error. This is similar to the way that assignments of invalid subcomponents are handled (see 13.9.1). 

Erroneous Execution

 {AI05-0033-1} {AI05-0262-1} 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.
Reason: {AI05-0033-1} {AI05-0262-1} 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.
Ramification: {AI05-0157-1} 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

For a standard storage pool, Free should actually reclaim the storage. 
Implementation Advice: For a standard storage pool, an instance of Unchecked_Deallocation should actually reclaim the storage.
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.
   {AI05-0157-1} 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. 
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.
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. 
30  The rules here that refer to Free apply to any instance of Unchecked_Deallocation.
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

{AI95-00416-01} 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

{AI05-0033-1} 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.
{AI05-0107-1} 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.
{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. 

Inconsistencies With Ada 2012

{AI12-0148-1} Corrigendum: Defined a "dangling reference", and specified that a dangling reference might designate some other existing object. This allows simple implementations of access values and reuse of object memory after deallocation. In prior versions of Ada, "=" between a dangling reference and an access to an existing object has to return False, even if the existing object and the object designated by the dangling reference are allocated in the same memory. A program that depended upon that could break with this revised rule. However, as a practical matter, almost all Ada implementations use simple implementations of access types that do not meet that requirement. So such a program would not work (consistently) on most Ada implementations; thus the change shouldn't break any existing programs - it just aligns the Standard with actual practice.
{AI12-0148-1} A side effect of this change is to allow an Ada implementation to detect dangling references in more places. This does not require any Ada implementation to change, and if the implementation does change, it just means that errors will be detected earlier.

Wording Changes from Ada 2012

{AI12-0148-1} Corrigendum: Clarified that deallocated objects cease to exist after finalization but before Deallocate is called. This is necessary to prevent erroneous execution from being triggered by the rules in 13.11 in the time between the end of finalization and the end of the call to the instance of Unchecked_Deallocation.

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