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 ::= access_to_object_definition | access_to_subprogram_definition
access_to_object_definition ::= access [general_access_modifier] subtype_indication
general_access_modifier ::= all | constant
access_to_subprogram_definition ::= access [protected] procedure parameter_profile | access [protected] function parameter_and_result_profile
access_definition ::= access subtype_mark
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. Finally, the current instance of a limited type, and a formal parameter or generic formal object of a tagged type are defined to be aliased. Aliased views are the ones that can be designated by an access value. If the view defined by an object_declaration is aliased, and the type of the object has discriminants, then the object is constrained; if its nominal subtype is unconstrained, then the object is constrained by its initial value. Similarly, if the object created by an allocator has discriminants, the object is constrained, either by the designated subtype, or by its initial 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-to-variable type; the subtype_mark denotes its designated subtype. An access_definition is used in the specification of an access discriminant (see 3.7) or an access parameter (see 6.1).
For each (named) access type, there is a literal null which has a null access value designating no entity at all. The null value of a named access type is the default initial value of the type. Other values of an access type are obtained by evaluating an attribute_reference for the Access or Unchecked_Access attribute of an aliased view of an object or non-intrinsic subprogram, or, in the case of a named access-to-object type, an allocator, which returns an access value designating a newly created object (see 3.10.2).
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.
A composite_constraint is compatible with an unconstrained access subtype if it is compatible with the designated subtype. 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.
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 general access-to-variable type (this happens as part of the initialization of an access parameter or access discriminant).
77 Access values are called pointers or references in some other languages.
78 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.
Examples of access-to-object types:
type Peripheral_Ref is 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
Copyright © 1992,1993,1994,1995 Intermetrics, Inc.
Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual