operator= in c++ -

possible duplicate:
trouble inheritance of operator= in c++

hello everyone, let's assume have 2 classes:

base{}; //inside have operator= derived{}; //inside don't have operator= 

why 1 working perfectly:

derived der1, der2; der1=der2;  //<-here know calls base::operator= 

and 1 not:

derived der1; base bas1; der1=bas1;  //<-here why can't call base::operator=? 

the implicitly declared copy assignment operator looks like

derived& operator=(const derived&); 

this implicitly declared function calls operator= each base class , member subobject (this why base class operator= overload called).

bas1 of type base, not derived, , there no implicit conversion base derived, hence doesn't work. need declare appropriate assignment operator in order support assigning object of type base object of type derived (this bit unusual though).