overriding with difference access specification c++ -

i came across question while taking ikm test. there base class 2 abstract methods private access specifier. there derived class overriding these abstract methods protected/public access specifier.

i never came across such thing overridden methods in derived class had different access specification. allowed ? if yes, comply "is a" relation between base , derived (i.e. safely substitutable).

could point me references can provide more details on such usages of classes ?

thank you.

as many of guys pointed out legal.

however, "is-a" part not simple. when comes "dynamic polymorphism" "is-a" relation holds, i.e. can super can derived instance.

however, in c++ have referred static polymorphism (templates, of time). consider following example:

class { public:     virtual int m() {         return 1;     } };  class b : public { private:     virtual int m() {         return 2;     } };  template<typename t> int fun(t* obj) {     return obj->m(); } 

now, when try use "dynamic polymorphism" seems ok:

a* = new a(); b* b = new b();  // dynamic polymorphism std::cout << a->m(); // ok std::cout << dynamic_cast<a*>(b)->m(); // ok - b instance conforms interface // std::cout << b->m(); fails compile due overriden visibility - expected since technically not violate is-a relationship 

... when use "static polymorphism" can "is-a" relation no longer holds:

a* = new a(); b* b = new b();  // static polymorphism std::cout << fun(a); // ok //std::cout << fun(b); // fails compile - b instance not conform interface @ compile time 

so, in end, changing visibility method "rather legal" that's 1 of ugly things in c++ may lead pitfall.