TITLE: Dynamic binding and deleting arrays of objects


PROBLEM: gray@celia.UUCP (Steve Gray)

I tried to post this once before but it didn't seem to take... so if this is a 
duplicate, my apologies:

I have the following:

struct A {
    A( void) {};
    virtual ~A( void) {};
};

struct B : A {
    int	*ptr;

    B() 
    {
	ptr = (int *) malloc( 10);
    };

    virtual ~B()
    {
	cfree( ptr);
    };
};

main()
{
A    *aPtr;
B    *bPtr;


    bPtr = new B[10];
    aPtr = (A *) bPtr;

    delete[10] aPtr;
}

This correctly calls the destructor for B.  However, on the second element in
the array ~B() crashes because the "this" pointer is wrong.  It has only been
advanced by sizeof(A) bytes (not sizeof(B) bytes).  This is bad.  Is it a
bug?

Am I doing something wrong?



RESPONSE: p j l @cs.uiuc.edu (Paul Lucas), 1 Dec 92
	  AT&T Bell Laboratories	

*****>	Cast <to A*> is unnecessary.


*****>	So is the '10' <in the delete call>.


*****>	No. <it is not a bug>


*****>	Yes; you're thinking that vectors are first-class objects in 
	C++ -- they're not.  The ++ operator is well-defined to
	increment by the size of object of the base type of the pointer,
	so:

		++aPtr;		// won't work
	
	won't work either.  If ++ doesn't work, for a pointer that
	really points to an object of a derived type, how can you
	expect delete to work?  

	If this worked, that would also imply heterogenous vectors
	(which is a contradiction since vectors are for storage of the
	same type).