Is there a way to call constructor with class instance pointer?

Question

We can call destructor explicitly through class pointer, why not constructor? Any idea?

#include <iostream>

class Con {
public:
    Con( int x ) : x( x ) {

    }

private:
    int x;
};

int main() {
    Con* c = new Con( 1 );
    //c->Con( 2 ); //illegal
    c->~Con(); // ok!
    delete c;
}

Thanks,

Answer 1

You can actually call it, it is just that the syntax is not that of calling a member method (of which the destructor is an special case), so it is not done with the member access operators. Rather you have to resort to the placement-new syntax:

Con c;
c.~Con();        // destroy, now c is not a Con anymore
new (&c) Con();  // recreate, now c is a Con again

As a particular case, in the C++0x proposal, that is actually used in one of the code examples, providing means to reuse a union as a different type in the event of an union containing non-POD elements:

union U {
   int i;
   float f;
   std::string s;
};

int main() {
   U u;
   new (&u.s) std::string( "foo" );
   u.s.~string();
   u.i = 5;
}

}

Answer 2

No. You cannot.

Con* c = new Con( 1 );
//c->Con( 2 ); //illegal

You've already called the constructor in the new expression.

By the time you've a valid pointer of type Con*, you've already created an object. And calling constructor on the "constructed" object doesn't even make sense. So why would C++ allow that?