c++ - 为命名空间中的类重载运算符 << 时发出警告

Question

我在重载 << 时收到警告(尽管有效)上课 Rectangle在命名空间 Shape .请注意，我使用 Clion 生成了过载，但仍然收到警告。

矩形.h

#include <ostream>

namespace Shape {

    class Rectangle {
    public:
        friend std::ostream &operator<<(std::ostream &os, const Rectangle &rectangle);
    };
}

矩形.cpp

#include "Rectangle.h"

using namespace Shape;

std::ostream& Shape::operator<<(std::ostream &os, const Rectangle &rectangle) {
    os << "rectangle";
    return os;
}

警告:

warning: 'std::ostream& Shape::operator<<(std::ostream&, const Shape::Rectangle&)' has not been declared within Shape
 std::ostream& Shape::operator<<(std::ostream &os, const Rectangle &rectangle) {
               ^~~~~

note: only here as a friend
         friend std::ostream &operator<<(std::ostream &os, const Rectangle &rectangle);
                              ^~~~~~~~

如何正确地做到这一点，这样我就不会收到警告？谢谢

Answer 1

我找到了!

您必须在命名空间中定义重载方法，而不是在类中。

#include <ostream>

    namespace Shape {

        class Rectangle {
        };
        std::ostream &operator<<(std::ostream &os, const Rectangle &rectangle);
    }

不是这样的:

#include <ostream>

    namespace Shape {

        class Rectangle {
        public:
            friend std::ostream &operator<<(std::ostream &os, const Rectangle &rectangle);
        };
    }