c++ - Access protected function from derived class -

i have following typical scenario, in want hide implementation details in child class, , expose through interface:

template <typename derived> class interface { public:     void a()     {         static_cast<derived*>(this)->_a();     } };  class implementation : public interface<implementation> { protected:     void _a()     {         /*         ...         */     } }; 

i think understand why doesn't work, , know declaring class interface friend of implementation solves it, when comes more complex hierarchies, multiple interfaces, , various levels of inheritance(as real case), things messy.

i avoid having declare friend class interface<implementation> in each class implements interface.

is there alternative nice-and-clean solution problem?

thanks!

how using virtual functions , polymorphism?

create object in child class , reassign interface class pointer or reference. create pure virtual function in interface class , define in child class.