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7.6 User-Defined Assignment and Finalization

Three kinds of actions are fundamental to the manipulation of objects: initialization, finalization, and assignment. Every object is initialized, either explicitly or by default, after being created (for example, by an object_declaration or allocator). Every object is finalized before being destroyed (for example, by leaving a subprogram_body containing an object_declaration, or by a call to an instance of Unchecked_Deallocation). An assignment operation is used as part of assignment_statements, explicit initialization, parameter passing, and other operations.

Default definitions for these three fundamental operations are provided by the language, but a controlled type gives the user additional control over parts of these operations. In particular, the user can define, for a controlled type, an Initialize procedure which is invoked immediately after the normal default initialization of a controlled object, a Finalize procedure which is invoked immediately before finalization of any of the components of a controlled object, and an Adjust procedure which is invoked as the last step of an assignment to a (nonlimited) controlled object.

Static Semantics

The following language-defined library package exists:

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package Ada.Finalization is pragma Preelaborate(Finalization); pragma Remote_Types(Finalization);

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type Controlled is abstract tagged private; pragma Preelaborable_Initialization(Controlled);

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procedure Initialize (Object : in out Controlled) is null; procedure Adjust (Object : in out Controlled) is null; procedure Finalize (Object : in out Controlled) is null;

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type Limited_Controlled is abstract tagged limited private; pragma Preelaborable_Initialization(Limited_Controlled);

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procedure Initialize (Object : in out Limited_Controlled) is null; procedure Finalize (Object : in out Limited_Controlled) is null; private ... -- not specified by the language end Ada.Finalization;

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A controlled type is a descendant of Controlled or Limited_Controlled. The predefined "=" operator of type Controlled always returns True, since this operator is incorporated into the implementation of the predefined equality operator of types derived from Controlled, as explained in 4.5.2. The type Limited_Controlled is like Controlled, except that it is limited and it lacks the primitive subprogram Adjust.

9.1/2

A type is said to need finalization if:

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it is a controlled type, a task type or a protected type; or

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it has a component that needs finalization; or

9.4/2

it is a limited type that has an access discriminant whose designated type needs finalization; or

9.5/2

it is one of a number of language-defined types that are explicitly defined to need finalization.

Dynamic Semantics

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During the elaboration or evaluation of a construct that causes an object to be initialized by default, for every controlled subcomponent of the object that is not assigned an initial value (as defined in 3.3.1), Initialize is called on that subcomponent. Similarly, if the object that is initialized by default as a whole is controlled, Initialize is called on the object.

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For an extension_aggregate whose ancestor_part is a subtype_mark denoting a controlled subtype, the Initialize procedure of the ancestor type is called, unless that Initialize procedure is abstract.

Initialize and other initialization operations are done in an arbitrary order, except as follows. Initialize is applied to an object after initialization of its subcomponents, if any (including both implicit initialization and Initialize calls). If an object has a component with an access discriminant constrained by a per-object expression, Initialize is applied to this component after any components that do not have such discriminants. For an object with several components with such a discriminant, Initialize is applied to them in order of their component_declarations. For an allocator, any task activations follow all calls on Initialize.

When a target object with any controlled parts is assigned a value, either when created or in a subsequent assignment_statement, the assignment operation proceeds as follows:

The value of the target becomes the assigned value.

The value of the target is adjusted.

To adjust the value of a (nonlimited) composite object, the values of the components of the object are first adjusted in an arbitrary order, and then, if the object is controlled, Adjust is called. Adjusting the value of an elementary object has no effect, nor does adjusting the value of a composite object with no controlled parts.

For an assignment_statement, after the name and expression have been evaluated, and any conversion (including constraint checking) has been done, an anonymous object is created, and the value is assigned into it; that is, the assignment operation is applied. (Assignment includes value adjustment.) The target of the assignment_statement is then finalized. The value of the anonymous object is then assigned into the target of the assignment_statement. Finally, the anonymous object is finalized. As explained below, the implementation may eliminate the intermediate anonymous object, so this description subsumes the one given in 5.2, “Assignment Statements”.

Implementation Requirements

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For an aggregate of a controlled type whose value is assigned, other than by an assignment_statement , the implementation shall not create a separate anonymous object for the aggregate. The aggregate value shall be constructed directly in the target of the assignment operation and Adjust is not called on the target object.

Implementation Permissions

An implementation is allowed to relax the above rules (for nonlimited controlled types) in the following ways:

For an assignment_statement that assigns to an object the value of that same object, the implementation need not do anything.

For an assignment_statement for a noncontrolled type, the implementation may finalize and assign each component of the variable separately (rather than finalizing the entire variable and assigning the entire new value) unless a discriminant of the variable is changed by the assignment.

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For an aggregate or function call whose value is assigned into a target object, the implementation need not create a separate anonymous object if it can safely create the value of the aggregate or function call directly in the target object. Similarly, for an assignment_statement, the implementation need not create an anonymous object if the value being assigned is the result of evaluating a name denoting an object (the source object) whose storage cannot overlap with the target. If the source object might overlap with the target object, then the implementation can avoid the need for an intermediary anonymous object by exercising one of the above permissions and perform the assignment one component at a time (for an overlapping array assignment), or not at all (for an assignment where the target and the source of the assignment are the same object).

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Furthermore, an implementation is permitted to omit implicit Initialize, Adjust, and Finalize calls and associated assignment operations on an object of a nonlimited controlled type provided that:

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any omitted Initialize call is not a call on a user-defined Initialize procedure, and

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any usage of the value of the object after the implicit Initialize or Adjust call and before any subsequent Finalize call on the object does not change the external effect of the program, and

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after the omission of such calls and operations, any execution of the program that executes an Initialize or Adjust call on an object or initializes an object by an aggregate will also later execute a Finalize call on the object and will always do so prior to assigning a new value to the object, and

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the assignment operations associated with omitted Adjust calls are also omitted.

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This permission applies to Adjust and Finalize calls even if the implicit calls have additional external effects.

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