c++ - 为成员函数中类的枚举成员重载运算符 <<

Question

如何为作为类成员的枚举重载 << 运算符。具体来说，我有以下代码:

#include <iostream>

using namespace std;

namespace foo {
    class bar {
    public:
        enum a { b, c, d};

        static void print() {
            cout << b << endl;
        }
    };

    ostream& operator<< (ostream& os, bar::a var) {

        switch (var) {
        case bar::b:
            return os << "b";
        case bar::c:
            return os << "c";
        case bar::d:
            return os << "d";
        }
        return os;
    }


}
int main() {
    foo::bar::print();

    return 0;
}

如何让打印函数打印“b”而不是“1”？

Answer 1

这是一个简单的解决方案:

#include <iostream>

using namespace std;

namespace foo {

    class bar {
    public:
        enum a { b, c, d};

        static void print();
    };

    ostream& operator<< (ostream& os, bar::a var) {

        switch (var) {
        case bar::b:
            return os << "b";
        case bar::c:
            return os << "c";
        case bar::d:
            return os << "d";
        }
        return os;
    }


    void bar::print() {
        cout << b << endl;
    }
}
int main() {
    foo::bar::print();

    return 0;
}

[编辑] 正如 aschepler 先前所述，您只需要确保 operator<<(ostream &, bar::a)在 bar::print 的定义之前可见.