Unchecked type conversions can be achieved in any language that permits code insertions or address clauses. Such conversions may, for example, be needed if a user wants to define his own allocation strategy for access types. In this case, conversions from integer to access values are necessary to define an ALLOCATE procedure and a converse FREE procedure.
From the point of view of programming management (and also of maintainability) it is desirable to provide a standard way to achieve such unchecked conversions. In this way the parts of a program that use such dangerous features are made easier to identify. The following generic library function is predefined to that effect.
generic type SOURCE is limited private; type TARGET is limited private; function UNCHECKED_CONVERSION(S : SOURCE) return TARGET;
A program unit that uses unchecked type conversions must mention this generic function in its with clauses. A possible scenario is indicated with the package LIST_HANDLING given below:
package LIST_HANDLING is type PLACE; type LIST is access PLACE; type PLACE is record SUCC, PRED : LIST; VALUE : INTEGER; end record; ... procedure ALLOCATE (X : out LIST); procedure FREE (X : in out LIST); ... pragma CONTROLLED(LIST); -- no garbage collection for LIST'STORAGE_SIZE use 0; -- new will not be usable end; with UNCHECKED_CONVERSION; package body LIST_HANDLING is function INT_TO_LIST is new UNCHECKED_CONVERSION(SOURCE => INTEGER, TARGET => LIST); ... procedure ALLOCATE(X : out LIST) is ADDRESS : INTEGER; begin ... -- Compute address, Then: X := INT_TO_LIST(ADDRESS); end; ... end LIST_HANDLING;
The function INT_TO_LIST is obtained within the body of LIST_HANDLING by instantiation of UNCHECKED_CONVERSION, and is used to convert an integer address into a LIST.
The programming environment may be able to control and restrict the programs that are allowed to get access to the function UNCHECKED_CONVERSION.