C++11引用限定符

#include<iostream>
#include<string>
using namespace std;
class Animal{
public:
Animal& operator=(const Animal& animal) &;//&引用限定符指定该赋值运算符只能用于左值 ;&&限定符指定赋值运算符只能用于右值 
private:
string name;
int weight;
};

Animal& Animal::operator=(const Animal& animal) &{
if(this!=&animal){
name=animal.name;
weight=animal.weight;
}
cout<<"Animal::operator=(const Animal& animal)&"<<endl;
return *this;
}
int main(){
Animal bird1,bird2,bird3;
move(bird1)=bird3;//error 
bird2=bird3;//ok
}