Question

I have a vector contains a,b,c,d,e
vec[2] is c, but will it automatically reorder after i delete/erase c ? i mean vec[2] is d after the operation.

Answer 1

Logically yes, as a vector is a dynamic array of element. You delete one, then everything that follows is moved.

In the same manner, the total length of the vector will decrease as you erase elements.

From cplusplus.com

This effectively reduces the vector size by the number of elements
removed, calling each element’s destructor before.

Because vectors keep an array format, erasing on positions other than
the vector end also moves all the elements after the segment erased to
their new positions, which may not be a method as efficient as erasing
in other kinds of sequence containers (deque, list).