Move Semantics

new bing的解释：

左值和右值

l-values live until the end of the scope
r-values live until the end of the line

一些实例，感觉以判断这行结束后，值还live与否判断是不是r-value

只有l-value才可以被通过&引用

但是在vector拷贝赋值时

template <typename T>
vector<T>& vector<T>::operator=(const vector<T>& other) {
if (&other == this) return *this;
_size = other._size;
_capacity = other._capacity;
delete[] _elems;
_elems = new T[other._capacity];
std::copy(other._elems, other._elems + other._size, _elems);
return *this;
}

额外的，为何要返回*this？https://blog.csdn.net/annjeff/article/details/108952294

但是这样导致我们不可能通过如下的方式便捷的定义和赋值数组

vector<int> vec;
vec = make_me_a_vec(123);

若是我们通过如下方式重载make_me_a_vec，这样会导致内存空间被无效利用，导致浪费：

vector<int> make_me_a_vec(int num) {
     vector<int> res;
     while (num != 0) {
         res.push_back(num%10);
         num /= 10;
}
return res;
}