Why is memory still accessible after std::map::clear() is called?

Question

I am observing strange behaviour of std::map::clear(). This method is supposed to call element's destructor when called, however memory is still accessible after call to clear().

For example:

struct A
{
  ~A() { x = 0; }
  int x;
};

int main( void )
{
  std::map< int, A * > my_map;
  A *a = new A();
  a->x = 5;
  my_map.insert( std::make_pair< int, *A >( 0, a ) );

  // addresses will be the same, will print 5
  std::cout << a << " " << my_map[0] << " " << my_map[0]->x << std::endl;

  my_map.clear();

  // will be 0
  std::cout << a->x << std::endl;

  return 0;
}

The question is, why is variable a still accessible after its destructor was called by map::clear()? Do I need to write delete a; after calling my_map.clear() or is it safe to overwrite the contents of a?

Thanks in advance for your help, sneg

Answer 1

If you store pointers on a map (or a list, or anything like that) YOU are the responsible for deleting the pointers, since the map doesn't know if they have been created with new, or not. The clear function only invokes destructors if you don't use pointers.

Oh, and one more thing: invoking a destructor (or even calling delete) doesn't mean the memory can't be accessed anymore. It only means that you will be accessing garbage if you do.