You would manually call destructors when you do your own memory management (like std::vector).  In this case, you would also need to manually call constructors too.  Here is a modified version of your test program that doesn’t double-destruct the object.

 

class A

{

public:

      A()

      {

            counter = 0;

      }

      ~A()

      {

            counter++;

            std::cout << "~A(): " << counter << std::endl;

      }

      int counter;

};

 

int main(int argc, char* argv[])

{

      A *a;

 

      //Allocate memory using C functions, which do not call constructors

      a = (A*) malloc (sizeof (A));

      cout << a << ' ' << a->counter << std::endl;

 

      //Manually call the constructor.  The (a) tells the compiler which pointer we are initializing

      new (a) A ();

      cout << a << ' ' << a->counter << std::endl;

 

      //Manually call the destructor

      a->~A();

      cout << a << ' ' << a->counter << std::endl;

 

      //Free memory using C functions, which do not call destructors

      free (a);

      return 0;

}