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3.10 Access Types

1
A value of an access type (an access value) provides indirect access to the object or subprogram it designates. Depending on its type, an access value can designate either subprograms, objects created by allocators (see 4.8), or more generally aliased objects of an appropriate type.
1.a
Discussion: A name denotes an entity; an access value designates an entity. The “dereference” of an access value X, written “X.all”, is a name that denotes the entity designated by X. 

Language Design Principles

1.b/3
{AI05-0299-1} Access values should always be well defined (barring uses of certain unchecked features of Clause 13). In particular, uninitialized access variables should be prevented by compile-time rules.

Syntax

2/2
{AI95-00231-01} access_type_definition ::= 
    [null_exclusionaccess_to_object_definition
  | [null_exclusionaccess_to_subprogram_definition
3
access_to_object_definition ::= 
    access [general_access_modifiersubtype_indication
4
general_access_modifier ::= all | constant
5
access_to_subprogram_definition ::= 
    access [protectedprocedure parameter_profile
  | access [protectedfunction  parameter_and_result_profile
5.1/2
{AI95-00231-01} null_exclusion ::= not null
6/2
{AI95-00231-01} {AI95-00254-01} {AI95-00404-01} access_definition ::= 
    [null_exclusionaccess [constantsubtype_mark
  | [null_exclusionaccess [protectedprocedure parameter_profile
  | [null_exclusionaccess [protectedfunction parameter_and_result_profile

Static Semantics

7/1
{8652/0012} {AI95-00062-01} There are two kinds of access types, access-to-object types, whose values designate objects, and access-to-subprogram types, whose values designate subprograms. Associated with an access-to-object type is a storage pool; several access types may share the same storage pool. All descendants of an access type share the same storage pool. A storage pool is an area of storage used to hold dynamically allocated objects (called pool elements) created by allocators[; storage pools are described further in 13.11, “Storage Management”].
8
Access-to-object types are further subdivided into pool-specific access types, whose values can designate only the elements of their associated storage pool, and general access types, whose values can designate the elements of any storage pool, as well as aliased objects created by declarations rather than allocators, and aliased subcomponents of other objects. 
8.a
Implementation Note: The value of an access type will typically be a machine address. However, a value of a pool-specific access type can be represented as an offset (or index) relative to its storage pool, since it can point only to the elements of that pool. 
9/5
{AI95-00225-01} {AI95-00363-01} {AI05-0053-1} {AI05-0142-4} {AI05-0277-1} {AI12-0228-1} A view of an object is defined to be aliased if it is defined by an object_declaration, component_definition, parameter_specification, or extended_return_object_declaration with the reserved word aliased, or by a renaming of an aliased view. In addition, the dereference of an access-to-object value denotes an aliased view, as does a view conversion (see 4.6) of an aliased view. A qualified_expression is an aliased view when the operand denotes an aliased view. The current instance of an immutably limited type (see 7.5) is defined to be aliased. Finally, a formal parameter or generic formal object of a tagged type is defined to be aliased. [Aliased views are the ones that can be designated by an access value.] 
9.a
Glossary entry: An aliased view of an object is one that can be designated by an access value. Objects allocated by allocators are aliased. Objects can also be explicitly declared as aliased with the reserved word aliased. The Access attribute can be used to create an access value designating an aliased object.
9.b
Ramification: The current instance of a nonlimited type is not aliased.
9.c
The object created by an allocator is aliased, but not its subcomponents, except of course for those that themselves have aliased in their component_definition.
9.d
The renaming of an aliased object is aliased.
9.e
Slices are never aliased. See 4.1.2 for more discussion. 
9.f/2
Reason: {AI95-00225-01} The current instance of a limited type is defined to be aliased so that an access discriminant of a component can be initialized with T'Access inside the definition of T. Note that we don't want this to apply to a type that could become nonlimited later within its immediate scope, so we require the full definition to be limited.
9.g
A formal parameter of a tagged type is defined to be aliased so that a (tagged) parameter X may be passed to an access parameter P by using P => X'Access. Access parameters are most important for tagged types because of dispatching-on-access-parameters (see 3.9.2). By restricting this to formal parameters, we minimize problems associated with allowing components that are not declared aliased to be pointed-to from within the same record.
9.h
A view conversion of an aliased view is aliased so that the type of an access parameter can be changed without first converting to a named access type. For example:
9.i
type T1 is tagged ...;
procedure P(X : access T1);
9.j
type T2 is new T1 with ...;
procedure P(X : access T2) is
begin
    P(T1(X.all)'Access);  -- hand off to T1's P
    . . .     -- now do extra T2-specific processing
end P;
9.k/2
This paragraph was deleted.{AI95-00363-01}
9.l/2
We considered making more kinds of objects aliased by default. In particular, any object of a by-reference type will pretty much have to be allocated at an addressable location, so it can be passed by reference without using bit-field pointers. Therefore, one might wish to allow the Access and Unchecked_Access attributes for such objects. However, private parts are transparent to the definition of “by-reference type”, so if we made all objects of a by-reference type aliased, we would be violating the privacy of private parts. Instead, we would have to define a concept of “visibly by-reference” and base the rule on that. This seemed to complicate the rules more than it was worth, especially since there is no way to declare an untagged limited private type to be by-reference, since the full type might by nonlimited.
9.m
Discussion: Note that we do not use the term “aliased” to refer to formal parameters that are referenced through multiple access paths (see 6.2). 
10
An access_to_object_definition defines an access-to-object type and its first subtype; the subtype_indication defines the designated subtype of the access type. If a general_access_modifier appears, then the access type is a general access type. If the modifier is the reserved word constant, then the type is an access-to-constant type[; a designated object cannot be updated through a value of such a type]. If the modifier is the reserved word all, then the type is an access-to-variable type[; a designated object can be both read and updated through a value of such a type]. If no general_access_modifier appears in the access_to_object_definition, the access type is a pool-specific access-to-variable type. 
10.a
To be honest: The type of the designated subtype is called the designated type
10.b
Reason: The modifier all was picked to suggest that values of a general access type could point into “all” storage pools, as well as to objects declared aliased, and that “all” access (both read and update) to the designated object was provided. We couldn't think of any use for pool-specific access-to-constant types, so any access type defined with the modifier constant is considered a general access type, and can point into any storage pool or at other (appropriate) aliased objects. 
10.c
Implementation Note: The predefined generic Unchecked_Deallocation can be instantiated for any named access-to-variable type. There is no (language-defined) support for deallocating objects designated by a value of an access-to-constant type. Because of this, an allocator for an access-to-constant type can allocate out of a storage pool with no support for deallocation. Frequently, the allocation can be done at link-time, if the size and initial value are known then. 
10.d
Discussion: For the purpose of generic formal type matching, the relevant subclasses of access types are access-to-subprogram types, access-to-constant types, and (named) access-to-variable types, with its subclass (named) general access-to-variable types. Pool-specific access-to-variable types are not a separately matchable subclass of types, since they don't have any “extra” operations relative to all (named) access-to-variable types. 
11
An access_to_subprogram_definition defines an access-to-subprogram type and its first subtype; the parameter_profile or parameter_and_result_profile defines the designated profile of the access type. There is a calling convention associated with the designated profile[; only subprograms with this calling convention can be designated by values of the access type.] By default, the calling convention is “protected” if the reserved word protected appears, and “Ada” otherwise. [See Annex B for how to override this default.] 
11.a
Ramification: The calling convention protected is in italics to emphasize that it cannot be specified explicitly by the user. This is a consequence of it being a reserved word. 
11.b/2
Implementation Note: {AI95-00254-01} For a named access-to-subprogram type, the representation of an access value might include implementation-defined information needed to support up-level references — for example, a static link. The accessibility rules (see 3.10.2) ensure that in a "global-display-based" implementation model (as opposed to a static-link-based model), a named access-to-(unprotected)-subprogram value need consist only of the address of the subprogram. The global display is guaranteed to be properly set up any time the designated subprogram is called. Even in a static-link-based model, the only time a static link is definitely required is for an access-to-subprogram type declared in a scope nested at least two levels deep within subprogram or task bodies, since values of such a type might designate subprograms nested a smaller number of levels. For the normal case of a named access-to-subprogram type declared at the outermost (library) level, a code address by itself should be sufficient to represent the access value in many implementations.
11.c
For access-to-protected-subprogram, the access values will necessarily include both an address (or other identification) of the code of the subprogram, as well as the address of the associated protected object. This could be thought of as a static link, but it will be needed even for global-display-based implementation models. It corresponds to the value of the “implicit parameter” that is passed into every call of a protected operation, to identify the current instance of the protected type on which they are to operate.
11.d
Any Elaboration_Check is performed when a call is made through an access value, rather than when the access value is first "created" via a 'Access. For implementation models that normally put that check at the call-site, an access value will have to point to a separate entry point that does the check. Alternatively, the access value could point to a "subprogram descriptor" that consisted of two words (or perhaps more), the first being the address of the code, the second being the elaboration bit. Or perhaps more efficiently, just the address of the code, but using the trick that the descriptor is initialized to point to a Raise-Program-Error routine initially, and then set to point to the "real" code when the body is elaborated.
11.e
For implementations that share code between generic instantiations, the extra level of indirection suggested above to support Elaboration_Checks could also be used to provide a pointer to the per-instance data area normally required when calling shared code. The trick would be to put a pointer to the per-instance data area into the subprogram descriptor, and then make sure that the address of the subprogram descriptor is loaded into a "known" register whenever an indirect call is performed. Once inside the shared code, the address of the per-instance data area can be retrieved out of the subprogram descriptor, by indexing off the "known" register.
11.f/2
This paragraph was deleted.{AI95-00344-01}
11.g/2
{AI95-00254-01} Note that access parameters of an anonymous access-to-subprogram type are permitted. Such parameters represent full “downward” closures, meaning that in an implementation that uses a per-task (global) display, the display will have to be passed as a hidden parameter, and reconstructed at the point of call. 
12/3
{AI95-00230-01} {AI95-00231-01} {AI95-00254-01} {AI05-0264-1} An access_definition defines an anonymous general access type or an anonymous access-to-subprogram type. For a general access type, the subtype_mark denotes its designated subtype; if the general_access_modifier constant appears, the type is an access-to-constant type; otherwise, it is an access-to-variable type. For an access-to-subprogram type, the parameter_profile or parameter_and_result_profile denotes its designated profile.
13/2
{AI95-00230-01} {AI95-00231-01} For each access type, there is a null access value designating no entity at all, which can be obtained by (implicitly) converting the literal null to the access type. [The null value of an access type is the default initial value of the type.] Nonnull values of an access-to-object type are obtained by evaluating an allocator[, which returns an access value designating a newly created object (see 3.10.2)], or in the case of a general access-to-object type, evaluating an attribute_reference for the Access or Unchecked_Access attribute of an aliased view of an object. Nonnull values of an access-to-subprogram type are obtained by evaluating an attribute_reference for the Access attribute of a nonintrinsic subprogram.
13.a/2
This paragraph was deleted.{AI95-00231-01}
13.b/2
This paragraph was deleted.{AI95-00231-01}
13.1/2
  {AI95-00231-01} A null_exclusion in a construct specifies that the null value does not belong to the access subtype defined by the construct, that is, the access subtype excludes null. In addition, the anonymous access subtype defined by the access_definition for a controlling access parameter (see 3.9.2) excludes null. Finally, for a subtype_indication without a null_exclusion, the subtype denoted by the subtype_indication excludes null if and only if the subtype denoted by the subtype_mark in the subtype_indication excludes null. 
13.c/2
Reason: {AI95-00231-01} An access_definition used in a controlling parameter excludes null because it is necessary to read the tag to dispatch, and null has no tag. We would have preferred to require not null to be specified for such parameters, but that would have been too incompatible with Ada 95 code to require.
13.d/2
{AI95-00416-01} Note that we considered imposing a similar implicit null exclusion for controlling access results, but chose not to do that, because there is no Ada 95 compatibility issue, and there is no automatic null check inherent in the use of a controlling access result. If a null check is necessary, it is because there is a dereference of the result, or because the value is passed to a parameter whose subtype excludes null. If there is no dereference of the result, a null return value is perfectly acceptable, and can be a useful indication of a particular status of the call. 
14/3
{8652/0013} {AI95-00012-01} {AI05-0264-1} [All subtypes of an access-to-subprogram type are constrained.] The first subtype of a type defined by an access_definition or an access_to_object_definition is unconstrained if the designated subtype is an unconstrained array or discriminated subtype; otherwise, it is constrained. 
14.a
Proof: The Legality Rules on range_constraints (see 3.5) do not permit the subtype_mark of the subtype_indication to denote an access-to-scalar type, only a scalar type. The Legality Rules on index_constraints (see 3.6.1) and discriminant_constraints (see 3.7.1) both permit access-to-composite types in a subtype_indication with such _constraints. Note that an access-to-access-to-composite is never permitted in a subtype_indication with a constraint.
14.b/2
Reason: {AI95-00363-01} Only composite_constraints are permitted for an access type, and only on access-to-composite types. A constraint on an access-to-scalar or access-to-access type might be violated due to assignments via other access paths that were not so constrained. By contrast, if the designated subtype is an array or discriminated type without defaults, the constraint could not be violated by unconstrained assignments, since array objects are always constrained, and discriminated objects are also constrained when the type does not have defaults for its discriminants. Constraints are not allowed on general access-to-unconstrained discriminated types if the type has defaults for its discriminants; constraints on pool-specific access types are usually allowed because allocated objects are usually constrained by their initial value. 

Legality Rules

14.1/2
  {AI95-00231-01} If a subtype_indication, discriminant_specification, parameter_specification, parameter_and_result_profile, object_renaming_declaration, or formal_object_declaration has a null_exclusion, the subtype_mark in that construct shall denote an access subtype that does not exclude null. 
14.c/2
To be honest: {AI95-00231-01} This means “directly allowed in”; we are not talking about a null_exclusion that occurs in an access_definition in one of these constructs (for an access_definition, the subtype_mark in such an access_definition is not restricted). 
14.d/2
Reason: {AI95-00231-01} This is similar to doubly constraining a composite subtype, which we also don't allow. 

Dynamic Semantics

15/2
{AI95-00231-01} A composite_constraint is compatible with an unconstrained access subtype if it is compatible with the designated subtype. A null_exclusion is compatible with any access subtype that does not exclude null. An access value satisfies a composite_constraint of an access subtype if it equals the null value of its type or if it designates an object whose value satisfies the constraint. An access value satisfies an exclusion of the null value if it does not equal the null value of its type.
16
The elaboration of an access_type_definition creates the access type and its first subtype. For an access-to-object type, this elaboration includes the elaboration of the subtype_indication, which creates the designated subtype.
17/2
{AI95-00230-01} {AI95-00254-01} The elaboration of an access_definition creates an anonymous access type. 
NOTES
18
86  Access values are called “pointers” or “references” in some other languages.
19
87  Each access-to-object type has an associated storage pool; several access types can share the same pool. An object can be created in the storage pool of an access type by an allocator (see 4.8) for the access type. A storage pool (roughly) corresponds to what some other languages call a “heap.” See 13.11 for a discussion of pools.
20
88  Only index_constraints and discriminant_constraints can be applied to access types (see 3.6.1 and 3.7.1). 

Examples

21
Examples of access-to-object types: 
22/4
{AI95-00433-01} {AI12-0056-1} type Frame is access Matrix;    --  see 3.6
type Peripheral_Ref is not null access Peripheral;  --  see 3.8.1
type Binop_Ptr is access all Binary_Operation'Class;
                                           -- general access-to-class-wide, see 3.9.1
23
Example of an access subtype: 
24
subtype Drum_Ref is Peripheral_Ref(Drum);  --  see 3.8.1
25
Example of an access-to-subprogram type: 
26
type Message_Procedure is access procedure (M : in String := "Error!");
procedure Default_Message_Procedure(M : in String);
Give_Message : Message_Procedure := Default_Message_Procedure'Access;
...
procedure Other_Procedure(M : in String);
...
Give_Message := Other_Procedure'Access;
...
Give_Message("File not found.");  -- call with parameter (.all is optional)
Give_Message.all;                 -- call with no parameters

Extensions to Ada 83

26.a
The syntax for access_type_definition is changed to support general access types (including access-to-constants) and access-to-subprograms. The syntax rules for general_access_modifier and access_definition are new. 

Wording Changes from Ada 83

26.b/3
{AI05-0190-1} We use the term "storage pool" to talk about the data area from which allocation takes place. The term "collection" is only used for finalization. ("Collection" and "storage pool" are not the same thing because multiple unrelated access types can share the same storage pool; see 13.11 for more discussion.) 

Inconsistencies With Ada 95

26.c/2
{AI95-00231-01} Access discriminants and noncontrolling access parameters no longer exclude null. A program which passed null to such an access discriminant or access parameter and expected it to raise Constraint_Error may fail when compiled with Ada 2005. One hopes that there no such programs outside of the ACATS. (Of course, a program which actually wants to pass null will work, which is far more likely.)
26.d/2
{AI95-00363-01} Most unconstrained aliased objects with defaulted discriminants are no longer constrained by their initial values. This means that a program that raised Constraint_Error from an attempt to change the discriminants will no longer do so. The change only affects programs that depended on the raising of Constraint_Error in this case, so the inconsistency is unlikely to occur outside of the ACATS. This change may however cause compilers to implement these objects differently, possibly taking additional memory or time. This is unlikely to be worse than the differences caused by any major compiler upgrade. 

Incompatibilities With Ada 95

26.e/2
{AI95-00225-01} Amendment Correction: The rule defining when a current instance of a limited type is considered to be aliased has been tightened to apply only to types that cannot become nonlimited. A program that attempts to take 'Access of the current instance of a limited type that can become nonlimited will be illegal in Ada 2005. While original Ada 95 allowed the current instance of any limited type to be treated as aliased, this was inconsistently implemented in compilers, and was likely to not work as expected for types that are ultimately nonlimited. 

Extensions to Ada 95

26.f/2
{AI95-00231-01} The null_exclusion is new. It can be used in both anonymous and named access type definitions. It is most useful to declare that parameters cannot be null, thus eliminating the need for checks on use.
26.g/2
{AI95-00231-01} {AI95-00254-01} {AI95-00404-01} The kinds of anonymous access types allowed were increased by adding anonymous access-to-constant and anonymous access-to-subprogram types. Anonymous access-to-subprogram types used as parameters allow passing of subprograms at any level. 

Wording Changes from Ada 95

26.h/2
{8652/0012} {AI95-00062-01} Corrigendum: Added accidentally-omitted wording that says that a derived access type shares its storage pool with its parent type. This was clearly intended, both because of a note in 3.4, and because anything else would have been incompatible with Ada 83.
26.i/2
{8652/0013} {AI95-00012-01} Corrigendum: Fixed typographical errors in the description of when access types are constrained.
26.j/2
{AI95-00230-01} The wording was fixed to allow allocators and the literal null for anonymous access types. The former was clearly intended by Ada 95; see the Implementation Advice in 13.11.
26.k/2
{AI95-00363-01} The rules about aliased objects being constrained by their initial values now apply only to allocated objects, and thus have been moved to 4.8, “Allocators”. 

Wording Changes from Ada 2005

26.l/3
{AI05-0053-1} {AI05-0277-1} Correction: The rule about a current instance being aliased now is worded in terms of immutably limited types. Wording was also added to make extended return object declarations that have the keyword aliased be considered aliased. This latter was a significant oversight in Ada 2005 — technically, the keyword aliased had no effect. But of course implementations followed the intent, not the letter of the Standard.
26.m/3
{AI05-0142-4} Explicitly aliased parameters (see 6.1) are defined to be aliased. 

Extensions to Ada 2012

26.n/5
{AI12-0228-1} Correction: A qualified_expression of an aliased object is now also aliased. 

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