Pitfalls of a vector

Every STL container has its own advantages and disadvantages. There is no single STL container that works better in all the scenarios. A vector internally uses an array data structure, and arrays are fixed in size in C/C++. Hence, when you attempt to add new values to the vector at the time the vector size has already reached its maximum capacity, then the vector will allocate new consecutive locations that can accommodate the old values and the new value in a contiguous location. It then starts copying the old values into the new locations. Once all the data elements are copied, the vector will invalidate the old location.

Whenever this happens, the vector insertion will take O(N) runtime complexity. As the size of the vector grows over time, on demand, the O(N) runtime complexity will show up a pretty bad performance. If you know the maximum size required, you could reserve so much initial size upfront in order to overcome this issue. However, not in all scenarios do you need to use a vector. Of course, a vector supports dynamic size and random access, which has performance benefits in some scenarios, but it is possible that the feature you are working on may not really need random access, in which case a list, deque, or some other container may work better for you.