c++ 类型转换 class 成员运算符问题

Question

有两个简单的类:

class X
{
    int value;

public:

    X() : value(0) {};

    X(int v) : value(v) {};

    X(const X& x) {     // Does not help!
        this->value = x.value;
    }

    X(const X&& x) noexcept { // Does not help!
        this->value = x.value;
    }

    X& operator=(const X& right)  { // Does not help!
        this->value = right.value;
        return *this;
    };

    X&& operator=(const X&& right) noexcept  { // Does not help!
        this->value = right.value;
        return std::move(*this);
    };

    bool operator==(X& right) const {
        return this->value == right.value;
    };

};

class Y
{
    int value;

public:

    Y() : value(0) {};

    operator X() const {

        return X(this->value);

    };  // Y objects may be converted to X
};

并举例说明我是如何使用它的：

int main()
{

X x1, x2;
Y y1;

x1 = x2;   // Compiles (using the X& operator=)

x1 = std::move(x2);   // Compiles (using the X&& operator=)

auto newv = (X)y1; // Compiles 

x1 == newv;   // Accepted!

x1 == (X)y1;   // Error!!!

} // END: main()

x1 == (X)y1 行生成错误 C2678: binary '==': no operator found which takes a left-hand operand of type 'X' (or there are no acceptable conversion)

and "没有运算符==匹配这些操作数，操作数类型是X==X

我尝试用 C++17 编译它。

如果像 "x1 == newv" 这样的行对编译器有好处，那有什么问题？

Answer 1

bool operator==(X& right) const

应该是

bool operator==(const X& right) const
//               ^^

否则不能用临时 X 调用它，(X)y1 是。

https://ideone.com/tN1w8T

另一个问题，与我评论中的编译器错误无关：
bool operator=(X& left) const 这是比较运算符 == 还是赋值 =？函数声明是混合的，实现是明显的对比。分配将是 X& operator=(const X& left) {value = left.value; return *this;}.

c++ 类型转换 class 成员运算符问题

c++ type conversion class member operator issue

c++

types

member

type-conversion

operator-keyword