c++ - 不能做运算符 << 重载

Question

我刚开始学习 CPP，但遇到运算符重载问题。我想重载 ostream 库中的“<<”运算符。这是我在头文件中的声明:

friend ostream& operator<<(ostream &out ,const Stone s);

那是cpp文件:

ostream& Stone:: operator<<( ostream &out , const Stone s ){

给定的错误是: 1 IntelliSense:类“Stone”没有成员“operator<<”

我做错了什么？

Answer 1

问题是你的cpp文件

std::ostream& Stone::operator<<(std::ostream& stream, const Stone s)
              ^^^^^
{
}

它不是成员函数，应该是:

// Also note the 2nd argument is `const &` (i.e., don't copy the object if you don't have to
std::ostream& operator<<(std::ostream& stream, const Stone& foo)
{

}

完整示例

如果您仍然遇到问题，您应该发布更多代码和/或您的错误。以下对我有用。

#include <iostream>

class Foo
{
public:
    Foo() : mValue(99) {}

    friend std::ostream& operator<<(std::ostream& stream, const Foo& foo);

private:
    int mValue;
};

std::ostream& operator<<(std::ostream& stream, const Foo& foo)
{
    return stream << foo.mValue;
}

int main()
{
    Foo f;
    std::cout << f << "\n";

    return 0;
}