如何在 C++ 中初始化类对象引用以模拟 NRVO？

我遇到了一个课程编程问题，它要求我初始化A ausing 引用传递（初始化A a中的func）。如何通过' 引用调用A' 构造函数？A

#include <iostream>
using namespace std;

class A
{
public:
    int x;
    A()
    {
        cout << "default constructor" << endl;
        x = 1;
    }

    A(int x)
    {   
        cout << "constructor with param = "  << x << endl;
        this->x = x;
    }

    ~A() {
        cout << "destructor" << endl;
    }

    void print() {
        cout << x << endl;
    }
};


void fun(A& a)
{
    a.A::A(10); // error!
    return;
}


int main()
{
    A a; 
    fun(a);
    a.print();
    return EXIT_SUCCESS;
}

这个问题是有背景的。老师要我们复制NRVO（命名返回值优化）的结果。

#include <iostream>
using namespace std;

class A
{
public:
    int x;
    A()
    {
        cout << "default constructor" << endl;
        x = 1;
    }

    A(int x)
    {   
        cout << "constructor with param = "  << x << endl;
        this->x = x;
    }

    ~A() {
        cout << "destructor" << endl;
    }

    void print() {
        cout << x << endl;
    }
};

A fun() {
    A a = A(10);
    return a;
}


int main()
{
    A a = fun();
    return EXIT_SUCCESS;
}

默认 g++ 编译器：

constructor with param = 10
destructor

如果我们关闭 NRVO：g++ test.cpp -fno-elide-constructors

constructor with param = 10
destructor
destructor
destructor
destructor

老师希望我们通过引用传递来复制NRVO（命名返回值优化）结果。