3.10 Access Types
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.
general_access_modifier ::= all
null_exclusion ::= not null
are two kinds of access types, access-to-object
types, whose values
designate objects, and access-to-subprogram
types, whose values
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
A storage pool is an area of storage used to
hold dynamically allocated objects (called pool elements
by allocators; storage pools are described further in 13.11
types are further subdivided into pool-specific
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
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
denotes 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.
defines an access-to-object type and its first subtype;
defines the designated subtype
of the access type. If a general_access_modifier
appears, then the access type is a general access type.
the modifier is the reserved word constant
, then the type is an
; 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
; 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.
defines an access-to-subprogram type and its first subtype;
defines the designated profile
of the access type.
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
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
which returns an access value designating a newly created object (see
), 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.
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.
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.
access value satisfies
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.
NOTE 1 Access values are called “pointers”
or “references” in some other languages.
NOTE 2 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
) for the access type. A storage pool
(roughly) corresponds to what some other languages call a “heap”.
for a discussion of pools.
Examples of access-to-object
Frame is access
Matrix; -- see 3.6
Peripheral_Ref is not null access
Peripheral; -- see 3.8.1
Binop_Ptr is access all
-- general access-to-class-wide, see 3.9.1
Example of an access
Peripheral_Ref(Drum); -- see 3.8.1
Example of an access-to-subprogram
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
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