c++ - 重载子类中的流插入运算符(<<)

Question

我有一个 A 类，我为其定义了一个(重载的)流插入运算符。我从这个 A 类公开派生了一个 B 类，它有一个额外的数据成员。因此，我需要为派生类重新定义重载的流插入运算符，我是这样做的:

#include <iostream>

using namespace std;

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

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

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

int main() {
    A a(10, 'x');
    B b(20, 'y', 5.23);
    cout << b;

    return 0;
}

Question-1: Is this an appropriate technique of doing so? If not, kindly let me know where I'm going wrong.

Question-2: On running, this program crashes. Why so?

Answer 1

支持这一点的一种常见方法是让重载运算符调用虚成员函数:

class A { 
public:
    virtual std::ostream &write(std::ostream &os) const { 
        // write self to os
        os << "A\n";
        return os;
    }
};

class B : public A { 
public:
    virtual std::ostream &write(std::ostream &os) const { 
        // write self to os
        // This can use the base class writer like:
        A::write(os);
        os << "B\n";
        return os;
    }
};

然后运算符重载只调用该成员函数:

std::ostream &operator<<(std::ostream &os, A const &a) {
    return a.write(os);
}

自 write是虚拟的，你正在传递 A通过 (const) 引用，这会根据实际类型调用正确的成员函数(即，如果对象确实是 A::write 的实例，则调用 A，如果它是 B::write 的实例，则调用 B)。

例如，我们可以这样练习:

int main() { 
    A a;
    B b;

    std::cout << a << "\n";
    std::cout << b << "\n";
}

...产生这样的输出:

A

A
B

在实际使用中，您可能想要制作 write功能 protected ，并使 operator<< A的 friend .