Question

如您所见，在两个实例上仅调用基类的流插入运算符的重载版本。我明白为什么会如此。这是因为没有动态绑定。但是，我该如何解决呢？

#include <iostream>

using namespace std;

class A {
    int i;
    char c;
public:
    A(int i = 0, char c = ' ') {
        this->i = i;
        this->c = c;
    }
    int getI() { return i; }
    char getC() { return c; }
    friend ostream& operator << (ostream&, A&);
};

class B : public A {
    double d;
public:
    B(int i = 0, char c = ' ', double d = 0.0) : A(i, c), d(d) {}
    friend ostream& operator << (ostream&, B&);
};

ostream& operator << (ostream& out, A& a) {
    out << "\nInteger: " << a.i << "\nCharacter: " << a.c << endl;
    return out;
}

ostream& operator << (ostream& out, B& b) {
    out << "\nInteger: " << b.getI() << "\nCharacter: " << b.getC() << "\nDouble: " << b.d << endl;
    return out;
}

int main() {
    A* a = new A (10, 'x');
    B* b = new B(20, 'y', 5.23);
    A* array[] = { a, b };
    cout << *(array[0]);
    cout << "\n______________________________\n";
    cout << *(array[1]);
    delete a;
    delete b;
    cin.get();
    return 0;
}

如何让cout << *(array[1]);调用重载的流插入运算符，该运算符将B的对象作为其参数之一？

Answer 1

您可以定义虚拟成员助手函数。

public void insert(String data)
    {
        Link link = new Link(data);

        if(head == null)
        {
            head = link;
            tail= link;
        }
        else
        {
            tail.next = link;
            tail = link;
            tail.next = head;

        }
    }


public void display()
    {


        // good implementation for display #2
        while(head != null)
        {
        //  System.out.println (head.data);
            head = head.next;
        }
    }

你甚至不再需要2 class A { public: virtual void toStream(ostream& out) const { out << "\nInteger: " << i << "\nCharacter: " << c << endl; } }; class B : public A { public: virtual void toStream(ostream& out) const { out << "\nInteger: " << getI() << "\nCharacter: " << getC() << "\nDouble: " << d << endl; } }; ostream& operator << (ostream& out, const A& a) { a.toStream(out); return out; }。

对于operator<<()，可以采取类似的技巧。

operator >> (istream& in, A& a)

Answer 2

如果A知道它将继承，您可以使用非虚拟接口：让操作员成为朋友，写一个受保护的（它不属于该类interface）执行写入的虚函数。

上传和重载流操作员

2 个答案: