Ada Resource Association
News and resources for the Ada programming language
Ada Reference ManualLegal Information
Contents   Index   References   Search   Previous   Next 

3.10 Access Types

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.


access_type_definition ::= 
  | [null_exclusionaccess_to_subprogram_definition
access_to_object_definition ::= 
    access [general_access_modifiersubtype_indication
general_access_modifier ::= all | constant
access_to_subprogram_definition ::= 
    access [protectedprocedure parameter_profile
  | access [protectedfunction  parameter_and_result_profile
null_exclusion ::= not null
access_definition ::= 
    [null_exclusionaccess [constantsubtype_mark
  | [null_exclusionaccess [protectedprocedure parameter_profile
  | [null_exclusionaccess [protectedfunction parameter_and_result_profile

Static Semantics

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”.
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. 
A view of an object is defined to be aliased if it is defined by an object_declaration or component_definition 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. The current instance of a limited tagged type, a protected type, a task type, or a type that has the reserved word limited in its full definition is also 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. 
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. 
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. 
 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.
 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. Non-null 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. Non-null values of an access-to-subprogram type are obtained by evaluating an attribute_reference for the Access attribute of a non-intrinsic subprogram..
   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. 
 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. 

Legality Rules

   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. 

Dynamic Semantics

 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.
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.
 The elaboration of an access_definition creates an anonymous access type. 
80  Access values are called “pointers” or “references” in some other languages.
81  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.
82  Only index_constraints and discriminant_constraints can be applied to access types (see 3.6.1 and 3.7.1). 


Examples of access-to-object types: 
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
Example of an access subtype: 
subtype Drum_Ref is Peripheral_Ref(Drum);  --  see 3.8.1
Example of an access-to-subprogram type: 
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

Contents   Index   References   Search   Previous   Next 
Ada-Europe Sponsored by Ada-Europe