构造函数forward完美转发问题

Question

这三种写法是正确的吗?有什么区别?

class A
{
public:
    std::vector<int> a;
    A(const A &that)
    {
        this->a=std::forward<const A&>(that).a;  //写法1
        //this->a=std::forward<const A>(that).a;  //写法2
        //this->a=std::forward<A>(that).a;  //写法3，会报错
    }
};

Answer 1

vector中类型是int啊