Object-oriented Software in Ada 95

15.7 Attributes 'Access and 'Unchecked_Access
Dynamic memory allocation 225
The access value of an object may only be taken if the object is declared at the same lexical level or lower than the
type declaration for the access value. If it is not, then a compile time error message will be generated when an
attempt is made to take the object’s access value. This is to prevent the possibility of holding an access value for
an object which does not exist. For example, in the following program:
procedure Main is
Max_Chs : constant := 7;
type Height_Cm is range 0 .. 300;
type Person is record
Name : String( 1 .. Max_Chs ); --Name as a String
Height : Height_Cm := 0; --Height in cm.
end record;
Mike : aliased Person := Person'("Mike ",156);
begin
declare
type P_Person is access all Person; --Access type
P_Human: P_Person;
begin
P_Human:= Mike'access; --OK
declare
Clive : aliased Person := Person'("Clive ", 160 );
begin
P_Human := Clive'Access;
end;
Put( P_Human.Name ); New_Line; --Clive no loner exists
P_Human := Mike'access;
end;
end Main;
a compile time error message is generated for the line:
--Change to Mike
P_Human := Clive'Access;
-- Compile time error
as the object Clive does not exist for all the scope of the type P_Person. In fact, there is a serious error in the
program, as when the line:
Put( P_Human.Name ); New_Line;
-- Clive no longer exists
is executed, the storage that p_human points to does not exist. Remember the scope of clive is the declare
block.
In some circumstances the access value of an object declared in an inner block to the access type declaration is
required. If this is so, then the compiler checking can be subverted or overridden by the use of
'Unchecked_Access. Thus, the following code can be written:
procedure Main is
-- Declaration of Person, P_Person, Mike etc.
begin
P_Human:= Mike'Access;
-- OK
declare
Clive : aliased Person := Person'("Clive ", 160 );
begin
P_Human := Clive'Unchecked_Access;
Put( P_Human.Name ); New_Line; -- Clive
end;
P_Human:= Mike'Access;
--Change to Mike
Put( P_Human.Name ); New_Line; --Mike
end Main;
Of course the compiler can no longer help the programmer in detecting possible inconstancies.
© M A Smith - May not be reproduced without permission