c++ - Alternative to casting return pointer of base class protected method? -

i came class uses protected nested struct, intending derived classes augment struct. end, declared virtual method allocating struct.

now, base class not-trivial amount of work in processsomedata , derived class reuse it.

which leads following:

class { public:     virtual void doprocessing(); // uses processsomedata()  protected:     struct somedata     {         virtual ~somedata() {};          // data members     };      virtual somedata* processsomedata(); // uses allocatesomedata()     virtual somedata* allocatesomedata(); };  class b : public { public:     virtual void doprocessing()     {         deriveddata* mydata =             static_cast<deriveddata*>(a::processsomedata()); // *** seems little suspect          // work on additional data members in mydata     }  protected:     struct deriveddata : public somedata     {         // more data members     };      virtual deriveddata* allocatesomedata(); }; 

---

because allocatesomedata overridden, know a::processsomedata returning somedata* pointing deriveddata, static_cast safe.

that said, feels little off should have cast base derived @ all, when else seems pretty kosher.
there better/proper way this, without using cast? or have re-design classes/structs?

it because compiler doesn't know sure processsomedata uses allocatesomedata create somedata struct. far compiler knows somedata, returned processsomedata might instance of somedata. deriveddata somedata not other way round.