3.10.2 Operations of Access Types
The attribute Access is used to create access values designating aliased objects and non-intrinsic subprograms. The accessibility rules prevent dangling references (in the absence of uses of certain unchecked features -- see Section 13).
Name Resolution Rules
For an attribute_reference with attribute_designator Access (or Unchecked_Access -- see 13.10), the expected type shall be a single access type; the prefix of such an attribute_reference is never interpreted as an implicit_dereference. If the expected type is an access-to-subprogram type, then the expected profile of the prefix is the designated profile of the access type.
The accessibility rules, which prevent dangling references, are written in terms of accessibility levels, which reflect the run-time nesting of masters. As explained in 7.6.1, a master is the execution of a task_body, a block_statement, a subprogram_body, an entry_body, or an accept_statement. An accessibility level is deeper than another if it is more deeply nested at run time. For example, an object declared local to a called subprogram has a deeper accessibility level than an object declared local to the calling subprogram. The accessibility rules for access types require that the accessibility level of an object designated by an access value be no deeper than that of the access type. This ensures that the object will live at least as long as the access type, which in turn ensures that the access value cannot later designate an object that no longer exists. The Unchecked_Access attribute may be used to circumvent the accessibility rules.
A given accessibility level is said to be statically deeper than another if the given level is known at compile time (as defined below) to be deeper than the other for all possible executions. In most cases, accessibility is enforced at compile time by Legality Rules. Run-time accessibility checks are also used, since the Legality Rules do not cover certain cases involving access parameters and generic packages.
Each master, and each entity and view created by it, has an accessibility level:
- The accessibility level of a given master is deeper than that of each dynamically enclosing master, and deeper than that of each master upon which the task executing the given master directly depends (see 9.3).
- An entity or view created by a declaration has the same accessibility level as the innermost enclosing master, except in the cases of renaming and derived access types described below. A parameter of a master has the same accessibility level as the master.
- The accessibility level of a view of an object or subprogram defined by a renaming_declaration is the same as that of the renamed view.
- The accessibility level of a view conversion is the same as that of the operand.
- For a function whose result type is a return-by-reference type, the accessibility level of the result object is the same as that of the master that elaborated the function body. For any other function, the accessibility level of the result object is that of the execution of the called function.
- The accessibility level of a derived access type is the same as that of its ultimate ancestor.
- The accessibility level of the anonymous access type of an access discriminant is the same as that of the containing object or associated constrained subtype.
- The accessibility level of the anonymous access type of an access parameter is the same as that of the view designated by the actual. If the actual is an allocator, this is the accessibility level of the execution of the called subprogram.
- The accessibility level of an object created by an allocator is the same as that of the access type.
- The accessibility level of a view of an object or subprogram denoted by a dereference of an access value is the same as that of the access type.
- The accessibility level of a component, protected subprogram, or entry of (a view of) a composite object is the same as that of (the view of) the composite object.
One accessibility level is defined to be statically deeper than another in the following cases:
- For a master that is statically nested within another master, the accessibility level of the inner master is statically deeper than that of the outer master.
- The statically deeper relationship does not apply to the accessibility level of the anonymous type of an access parameter; that is, such an accessibility level is not considered to be statically deeper, nor statically shallower, than any other.
- For determining whether one level is statically deeper than another when within a generic package body, the generic package is presumed to be instantiated at the same level as where it was declared; run-time checks are needed in the case of more deeply nested instantiations.
- For determining whether one level is statically deeper than another when within the declarative region of a type_declaration, the current instance of the type is presumed to be an object created at a deeper level than that of the type.
The accessibility level of all library units is called the library level; a library-level declaration or entity is one whose accessibility level is the library level.
The following attribute is defined for a prefix X that denotes an aliased view of an object:
- X'Access yields an access value that designates the object denoted by X. The type of X'Access is an access-to-object type, as determined by the expected type. The expected type shall be a general access type. X shall denote an aliased view of an object, including possibly the current instance (see 8.6) of a limited type within its definition, or a formal parameter or generic formal object of a tagged type. The view denoted by the prefix X shall satisfy the following additional requirements, presuming the expected type for X'Access is the general access type A with designated type D:
- If A is an access-to-variable type, then the view shall be a variable; on the other hand, if A is an access-to-constant type, the view may be either a constant or a variable.
- The view shall not be a subcomponent that depends on discriminants of a variable whose nominal subtype is unconstrained, unless this subtype is indefinite, or the variable is aliased.
- If A is a named access type and D is a tagged type, then the type of the view shall be covered by D; if A is anonymous and D is tagged, then the type of the view shall be either DClass or a type covered by D; if D is untagged, then the type of the view shall be D, and As designated subtype shall either statically match the nominal subtype of the view or be discriminated and unconstrained;
- The accessibility level of the view shall not be statically deeper than that of the access type A. 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.
A check is made that the accessibility level of X is not deeper than that of the access type A. If this check fails, Program_Error is raised.
If the nominal subtype of X does not statically match the designated subtype of A, a view conversion of X to the designated subtype is evaluated (which might raise Constraint_Error -- see 4.6) and the value of X'Access designates that view.
The following attribute is defined for a prefix P that denotes a subprogram:
- P'Access yields an access value that designates the subprogram denoted by P. The type of P'Access is an access-to-subprogram type (S), as determined by the expected type. The accessibility level of P shall not be statically deeper than that of S. 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. The profile of P shall be subtype-conformant with the designated profile of S, and shall not be Intrinsic. If the subprogram denoted by P is declared within a generic body, S shall be declared within the generic body.
81 The Unchecked_Access attribute yields the same result as the Access attribute for objects, but has fewer restrictions (see 13.10). There are other predefined operations that yield access values: an allocator can be used to create an object, and return an access value that designates it (see 4.8); evaluating the literal null yields a null access value that designates no entity at all (see 4.2).
82 The predefined operations of an access type also include the assignment operation, qualification, and membership tests. Explicit conversion is allowed between general access types with matching designated subtypes; explicit conversion is allowed between access-to-subprogram types with subtype conformant profiles (see 4.6). Named access types have predefined equality operators; anonymous access types do not (see 4.5.2).
83 The object or subprogram designated by an access value can be named with a dereference, either an explicit_dereference or an implicit_dereference. See 4.1.
84 A call through the dereference of an access-to-subprogram value is never a dispatching call.
85 The accessibility rules imply that it is not possible to use the Access attribute to implement downward closures -- that is, to pass a more-nested subprogram as a parameter to a less-nested subprogram, as might be desired for example for an iterator abstraction. Instead, downward closures can be implemented using generic formal subprograms (see 12.6). Note that Unchecked_Access is not allowed for subprograms.
86 Note that using an access-to-class-wide tagged type with a dispatching operation is a potentially more structured alternative to using an access-to-subprogram type.
87 An implementation may consider two access-to-subprogram values to be unequal, even though they designate the same subprogram. This might be because one points directly to the subprogram, while the other points to a special prologue that performs an Elaboration_Check and then jumps to the subprogram. See 4.5.2.
Example of use of the Access attribute:
Martha : Person_Name := new Person(F); -- see 3.10.1 Cars : array (1..2) of aliased Car; ... Martha.Vehicle := Cars(1)'Access; George.Vehicle := Cars(2)'Access;
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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual