Object-oriented Software in Ada 95

Containers 255
When an instance of class List is assigned, only the direct storage contained in the record list will be
copied. This will not physically duplicate the storage contained in the list, but only copy the pointers to the list.
When a controlled object is assigned, the procedure Adjust is called after the assignment has been made. The
procedure Adjust is used to perform any additional actions required on an assignment. The exact effect of
assigning a controlled object is as follows:
Assignment of controlled objects
A := B;
Actions that take place on assignment
Anon := B;
Adjust( Anon );
Finalize( A );
A := Anon;
Adjust( a );
Finalize( Anon );
Action on assignment
Anon := B
Adjust(Anon);
Finalize(A);
A := Anon;
Adjust(A);
Finalize(Anon);
Commentary
Make a temporary anonymous copy anon.
Adjustments required to be made after copying the
direct storage of the source object B to Anon.
Finalize the target of the assignment.
Perform the physical assignment of the direct
components of the anon object.
Adjustments required to be made after copying the
direct storage of the Anon object.
Finalize the anonymous object Anon.
Note: If the object’s storage does not overlap, which will be the usual case, then the compiler may implement
the following optimization:
Finalize(A); A := B; Adjust(A);
Look at the effect of the assignment A := A; to see why this optimization may not be performed when
the object’s storage overlaps.
If the source and target are the same, then the operation may be skipped.
The procedure Adjust is used to create a new duplicate copy of the storage in the list. The contents of the
target in the assignment are updated to point to this newly created copy. Hence, there are now two identical copies
of the linked list:
• The linked list in the Target.
• The linked list in the Source .
procedure Adjust( The:in out List ) is
Cur : P_Node := The.First_Node; --Original list
Lst : P_Node := null; --Last created node
Prv : P_Node := null; --Previously created node
Fst : P_Node := null; --The first node
begin
while Cur /= null loop
Lst := new Node'( Prv, Cur.Item, null );
if Fst = null then Fst := Lst; end if;
if Prv /= null then Prv.Next := Lst; end if;
Prv := Lst;
Cur := Cur.Next; --Next node
end loop;
The.First_Node := Fst; --Update
The.Last_Node := Lst;
end Adjust;
© M A Smith - May not be reproduced without permission