c++primer一道习题-多态

原帖子地址：http://topic.csdn.net/u/20090829/11/e6ffa5bf-98bf-4c4a-84f7-bc296f3e8bde.html?seed=1086748632&r=59432039#r_59432039

假定Derived继承Base，并且Base将下面的函数定义为虚函数；假定Derived打算定义自己的这个虚函数的版本，确定在Derived中哪个声明是错误的，并指出为什么错 
a、Base* Base::copy(Base*); 
  Base* Derived::copy(Derived*); 
b、Base* Base::copy(Base*); 
  Derived* Derived::copy(Base*); 
c、ostream& Base::print(int, ostream& = cout ); 
  ostream& Derived::print(int, ostream&); 
d、void Base::eval() const; 
  void Derived::eval();  

 

a错误

#include <iostream>using namespace std;class Base{ public: virtual void Base::print(Base*) { cout<<"base"<<endl; }};class Derived:public Base{ public: void Derived::print(Derived*) { cout<<"derived"<<endl; }};int main(){ Base * b = new Derived(); b->print(new Derived()); return 0;}

 

c正确 
#include <iostream> using namespace std; class Base { public: virtual ostream& Base::print(int, ostream& = cout ) = 0; }; class Derived:public Base { public: ostream& Derived::print(int, ostream& = cout ) { return cout; } }; int main(){ Derived d; return 0; }

d错误 
#include <iostream> using namespace std; class Base { public: virtual void eval() const = 0; }; class Derived:public Base { public: void eval(){} }; int main(){ Derived d; return 0; }
error C2259: “Derived”: 不能实例化抽象类 

可以这样理解：因为const函数可以区分函数的重载，而默认参数不可以区分。