vector push_back 超过 capacity 时产生的内存拷贝问题

根据 C++ Reference 的官方解释：http://www.cplusplus.com/reference/vector/vector/

Just like arrays, vectors use contiguous storage locations for their elements, which means that their elements can also be accessed using offsets on regular pointers to its elements, and just as efficiently as in arrays. But unlike arrays, their size can change dynamically, with their storage being handled automatically by the container.
Internally, vectors use a dynamically allocated array to store their elements. This array may need to be reallocated in order to grow in size when new elements are inserted, which implies allocating a new array and moving all elements to it. This is a relatively expensive task in terms of processing time, and thus, vectors do not reallocate each time an element is added to the container.

从此可以看出，std::vector 具有以下特点：

• 内部使用数组的方式进行数据存储，内存连续</