c++ - 运算符 << 重载继承，为什么我从基类而不是子类获取输入？

Question

我有一个类，我们称它为A ，它只有一个字段，aa .我还有一个类(class)，B , 它继承类 A , 而不是 aa领域，它也有自己的领域，bb .现在，我重载了 operator << ( cout ) 两个类。我尝试使用多态性，但多态性似乎无法与运算符一起正常工作。我的代码只显示了 aa使用运算符时的字段 cout用于显示对象 obj .

我的意思是，我是否总是需要添加 virtual word 重载子类中基类的函数？如果是这样，我应该如何对运营商做同样的事情，运营商不能是虚拟的......

#include <iostream>
#include <ostream>
using namespace std;

class A
{
    protected :
        int aa;

    public:
        A(int aa) {this->aa = aa;}
        ~A(){}
        friend ostream &operator<<(ostream &os, const A& obj);
        virtual void show() {cout << "a = " << aa << "\n"; }
};

ostream &operator<<(ostream &os, const A& obj)
{
    for(int i=0; i<80; i++)
        os << "-";
    os << "\n";

    os << "a = " << obj.aa << "\n";

    for(int i=0; i<80; i++)
        os << "-";
    os << "\n";

    return os;
}

class B : public A
{
    private :
        int bb;

    public:
        B(int aa, int bb) : A(aa) {this->bb = bb;}
        ~B(){}
        friend ostream &operator<<(ostream &os, const B& obj);
        void show() {cout << "a = " << aa << "\n"; cout << "b = " << bb << "\n";}
};

ostream &operator<<(ostream &os, const B& obj)
{
    for(int i=0; i<80; i++)
        os << "-";
    os << "\n";

    os << "a = " << obj.aa << "\n";
    os << "b = " << obj.bb << "\n";

    for(int i=0; i<80; i++)
        os << "-";
    os << "\n";

    return os;
}


int main()
{
    A *obj = new B(2,3);
    cout << *obj;
    obj->show();
    delete obj;

    return 0;
}

Answer 1

I mean, do I always need to add the virtual word to overload the function from the base class in a child class? If so, how should I do the same with operators, operators can't be virtual ...

当然可以。但是只有成员函数可以是虚拟的，并且这些运算符不是成员函数 - 它们是全局函数。

在这种情况下，您不能使它们成为成员函数(因为第一个参数不是您的类的实例)。但是您可以创建一个虚拟成员函数并让运算符(operator)调用它:

class A
{
protected:
    virtual void print(ostream &);
public:
    friend ostream &operator<<(ostream &os, const A& obj);
    // ... other stuff ...
};

ostream &operator<<(ostream &os, const A& obj)
{
    obj.print(os);
    return os;
}

然后覆盖 print而不是 operator<< .