c++ - Virtual assignment operator not allowing static_cast -


i have following mwe code:

#include <algorithm>  class base{ public:     int basemember;     friend void swap(base& in, base& out)     {         using std::swap;         swap(in.basemember, out.basemember);     }     virtual base& operator=(base obj)     {         swap(*this, obj);         return *this;     }     base() : basemember(1)     {            } }; class derived : public base { public:     int derivedmember;     friend void swap(derived& in, derived& out)     {         using std::swap;         swap(in.derivedmember, out.derivedmember);         swap(static_cast<base&>(in), static_cast<base&>(out));     }     virtual base& operator=(base obj)     {         swap(*this, static_cast<derived&>(obj));         return *this;     }     derived() : base(), derivedmember(2)     {     } };  int main() {     base *b1 = new derived();     base *b2 = new derived();     *b1 = *b2;     delete b1;     delete b2; }

i have 2 base pointers pointing derived data. assignment of contents of pointers. since base class' assignment operator virtual, polymorphism kicks in , assignment operator of derived class (with same signature) called instead.

however, static_cast transform source derived object fails. or, well, transforms derived object, derivedmember garbage (after being set in constructor).

how can avoided? how can assignment between derived contents of 2 base pointers done?

your code has typo , inherently unsafe behavior. typo here:

virtual base& operator=(base obj) // <-- should base& {     swap(*this, static_cast<derived&>(obj));     return *this; }

if fix this, code should work in simple example provided. still fail @ large, because how guarantee argument passed operator= in fact of derived type?


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