Derived中派生类中的声明
C++Primer p502 题15.25
假定Derived继承Base，并且Base将下面的函数定义虚函数，Derived定义自己的这个虚函数的版本，确定在Derived中哪个声明是错的，并指出为什么？
///(a)
Base* Base::copy(Base*);
Base* Derive::copy(Derive*);
///(b)
Base* Base::copy(Base*);
Derive* Derive::copy(Derive*);
///(c)
ostream& Base::print(int, ostream &= cout);
ostream& Derived::print(int, ostream&);
///(d)
void Base::eval()const;
void Derive::eval();
