c++ - Override function that return base type -

c++ - Override function that return base type -

i have 2 classes. base of operations class parent , derived class child. class parent has pure virtual function returns class type. how can override in derived class?

class parent { public: virtual parent* overrideme(parent* func) = 0; }; class kid : public parent { public: child* overrideme(child* func) override; };

i tried child* overrideme(child* func) override; end error not override base of operations class member.

if c++ had total covariance , contravariance support, right relationship contravariant in input , covariant in output. namely:

struct organism { }; struct animal : organism { virtual animal* overrideme(animal* ) = 0; }; struct dog : aniaml { dog* overrideme(organism* ) override { ... } ↑↑↑ ↑↑↑↑↑↑↑↑ covariant contravariant };

it seems little unintuitive, create sense. if expecting animal*, should able handle animal* (of dog* qualifies). conversely, if doing operation on animal*, need operation can take animal* - , operation takes organism* qualifies on front.

note if input covariant, break type system. consider like;

animal* = new dog; a->overrideme(new cat);

if dog::overrideme allowed take dog*, fail - cat* not dog*! allowed take animal* ... or more generic (e.g. organism*), since of work fine.

c++ not have back upwards contravariance in input, covariance in output. write either:

dog* overrideme(animal* ) override { ... }

or:

animal* overrideme(animal* ) override { .... }

but nil else.

c++ inheritance interface abstract-class pure-virtual