Re: delete [] of Derived objects through Base object which has virtual dtor

James Kanze <>
Tue, 29 Apr 2008 02:38:59 -0700 (PDT)
On Apr 29, 4:32 am, ", India"
<> wrote:


class Base
virtual ~Test() { ... }
// ...

class Derived : public Base
virtual ~Derived() { ... }
// ...

int main()
      Base* base_ptr = new Derived[10]();
      delete [] base_ptr;
      return EXIT_SUCCESS;

If the Base class dtor is not not virtual, 'delete []
base_ptr' has undefined behaviour.

Will 'delete [] base_ptr' call each Derived class dtor because
the Base::~Base() is virtual ? Is the deletion in the above
code valid ?. Or does this also invoke undefined behaviour ?

As Kai-Uwe has pointed out, it is undefined behavior. More
generally, although the compiler will tranquilly convert
Derived* to Base* even if Derived* points to an array (because
it cannot know this), the resulting pointer can only be used as
a pointer to the first individual object. In you case, for
example, no only is the delete[] undefined behavior, but any use
of base_ptr to access the allocated array (e.g. base_ptr[1])
would be as well.

In general, don't use array new; prefer std::vector. And don't
try to make array elements polymorphic; it doesn't work. (Array
elements are values, and polymorphism only works through
pointers or references. If you need an array of polymorphic
types, you must use std::vector< Base* >, allocating and
deallocating each one manually.)

James Kanze (GABI Software)
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