C++ 操作员过载时发出警告<&书信电报；对于命名空间中的类

当重载

时，我会收到一条警告（尽管有效），只需删除
形状：：
：

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

我找到了

您必须在名称空间中而不是在类中定义重载方法

#include <ostream>

    namespace Shape {

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

#包括
名称空间形状{
类矩形{
};
std：：ostream&operator非常感谢你，我猜不出这个！！如果你这样做，那么这个
操作符是有效的，但是如果我调用矩形函数，它会产生一个错误，就像这样
cout@Brian是的，但是我怎么能不得到警告呢？在函数周围添加
名称空间形状{
..
}
。
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);
                              ^~~~~~~~

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

namespace Shape {

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

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

#include <ostream>

    namespace Shape {

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

#include <ostream>

    namespace Shape {

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