The const-ness of a member function is an integral part of its type. The following class defines two member functions, f(), g(), which have the same return type and parameter list:
class A{public: int f(int n) { return 0;} int g(int n) const {return 0;}};
However, g() is a const member function, whereas f() isn’t. Therefore, their types are not the same:
int (A::*pmf)(int)=&A::f;int (A::*pcmf) (int)const=&A::g; // ptr to const member
You can use pcmf to call a member function of a const object. Trying to use pmf in this context will fail because it’s not pointing to a const member function:
const A *p=new A; // p is a pointer to a const object(p->*pcmf)(5); // fine, using ptr to const member(p->*pmf)(5); // error, not a pointer to a const member
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