c++11 - C++ invalid conversion error using covariant return types with virtual functions -

in following code, can assign return of d::clone() pointer b, not pointer d. is possible return actual polymorphic type call of base pointer?

struct b {     virtual b * clone() { return this; } };  struct d : b {     d * clone()     {         std::cout << std::is_same<decltype(this), d *>::value << std::endl;         return this;     } };  int main() {     b * b = new d();     b * bb = b->clone(); // prints "1"     std::cout << std::is_same<decltype(b->clone()), d *>::value << std::endl; // prints "0"     d * d = b->clone(); // error: invalid conversion b * d * (gcc 5.1) } 

no. call of clone() on base b class of d return d* cast b*.

you can reverse doing static_cast<d*> if absolutely certain, or dynamic_cast<d*> if not certain. if certain, should make variable b d*.

in c++, should encode know state of program @ compile time regardless of run time situation types. storing d* b* b, telling compiler not "know" data being pointed "is d". instead, saying "the code using b*b should valid pointer-to-b. d* d = b->clone(); not guaranteed valid every pointer-to-b.