c++ - Invalid covariant type with CRTP clonable class -


i'm trying implement clonable class crtp. however, need have abstract class have pure virtual clone method, overridden child classes. make happen, need clone function return covariant return type. made code below, , compiler shout @ me error:

main.cpp:12:5: error: return type of virtual function 'clone' not covariant return type of function overrides ('b *' not derived 'abstractclonable *')

the class 'b' seems child class of abstractclonable, , 2 way! how can solve this? thank much. tried both clang 3.6 , gcc 4.9.2

struct abstractclonable {     virtual abstractclonable* clone() const = 0; };  template<typename t> struct clonable : virtual abstractclonable {     t* clone() const override {         return new t{*dynamic_cast<const t*>(this)};     } };  struct : virtual abstractclonable {  };  struct b : a, clonable<b> {  };

even if b indeed derived clonable<b>, problem here clonable<b> construction not valid, defines 

b* clone() const override

which of course not override of abstractclonable::clone(), since compiler doesn't see b @ point child of abstractclonable. believe issue lays in fact compiler cannot build clonable<b> base of b.

a workaround (but not same want) define

clonable* clone() const override

in clonable. mentioned in comment, can define free function 

template<typename t>  t* clone(const t* object)  {      return static_cast<t*>(object->clone());  }

related: derived curiously recurring templates , covariance


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