rvalue作为构造对象的初始化器

我是编程新手.对不起,我的英语不好.我试图使用rvalue作为初始对象的初始化器.因此,根据代码,它将打印出使用的构造函数和赋值运算符.但结果是对象"what2"和"what3",那些不打印任何东西.这是代码:

#include 
using namespace std;
class X{
public:
    int x;
    X()= default;
    X(int num):x{num}{}
    X(const X& y){
        x = y.x;
        std::cout << "copy constructor" << std::endl;
        std::cout << x << std::endl;

    }
    X& operator=(const X& d){
        x = d.x;
        std::cout << "copy assignment" << std::endl;
        return *this;
    }
    X(X&& y){
        x = y.x;
        std::cout << "move constructor" << std::endl;
    }
    X& operator=(X&& b){
        x = b.x;
        std::cout << "move assignment" << std::endl;
        return *this;
    }
};

X operator +(const X& a,const X& b){
    X tmp{};
    tmp.x = a.x +b.x;
    return tmp;
}

int main(int argc, const char * argv[]) {
    X a{7} , b{11};
    X what1;
    cout << "---------------" << endl;
    what1 = a+b;
    cout << "---------------" << endl;
    X what2{a+b};
    cout << "---------------" << endl;
    X what3 = a+b;
    cout << "---------------" << endl;
    std::cout << what1.x << std::endl;
    std::cout << what2.x << std:: endl;
    std::cout <

输出是:

---------------
move assignment
---------------
---------------
---------------
18
18
18
Program ended with exit code: 0


只有"what1"正确使用赋值.那么,我如何使用rvalue初始化一个对象？并使用operator =来初始化一个对象？非常感谢你.