之前提到vector中不断的push_back，会进行内存的重新自动分配的问题 （详见：https://blog.csdn.net/hl_zzl/article/details/84798804 ）

为了避免重新分配内存带来的问题，vector提供了reserve函数。

std::vector::reserve

Request a change in capacity Requests that the vector capacity be at least enough to contain n elements. If n is greater than the current vector capacity, the function causes the container to reallocate its storage increasing its capacity to n (or greater). In all other cases, the function call does not cause a reallocation and the vector capacity is not affected. This function has no effect on the vector size and cannot alter its elements.

reserve的作用是更改vector的容量（capacity），使vector至少可以容纳n个元素。 如果n大于vector当前的容量，reserve会对vector进行扩容。其他情况下都不会重新分配vector的存储空间

说明：在main中声明了两个vector，vecInt为默认初始化，vecIntB使用capacity初始化其容量为100。分别对vetIntA和vecIntB进行同样的操作： ①把0~99依次push_back到vector中， ②在push_back的过程中观察vector的容量capacity是否发生变化。