右值引用主要是为了优化。
在函数返回值没有打开-fno-elide-constructors时，函数返回值会调用拷贝构造函数

class X
{
public:
	X()
	{
		cout << "X ctor" << endl;
	}

	X(const X& x)
	{
		cout << "X copy ctor" << endl;
	}

	~X()
	{
		cout << "X dtor" << endl;
	}
};

X makeX()
{
	X x1;
	return x1;
}

int main()
{
	X x = makeX();
	return 0;
}

参数是右值引用，如果传的参数是左值，会编译不通过

void printName(string&& name)
{
	cout << name << endl;
}

int main()
{
	string firstName = "wang", lastName = "wenhao";

	string fullName = firstName + lastName;
	printName(fullName);

	return 0;
}

需要改成printName(firstName + lastName)