Template class implicit copy constructor issues

Question

Stepping through my program in gdb, line 108 returns right back to the calling function, and doesn't call the copy constructor in class A, like (I thought) it should:

template <class S> class A{
    //etc...
    A( const A & old ){
        //do stuff...
    }
    //etc...
};

template <class T> class B{
    //etc...
    A<T> ReturnsAnA(){
        A<T> result;
        // do some stuff with result
        return result; //line 108
    }
    //etc...
};

Any hints? I've banged my head against the wall about this for 4 hours now, and can't seem to come up with what's happening here.

Answer 1

(Named) return value optimization is in effect. Your copy constructor is being removed as an optimization (this is permitted by the standard although results in different behavior).

See also http://stackoverflow.com/questions/1394229/understanding-return-value-optimization-and-returning-temporaries-c.

(Templates have nothing to do with this.)