C++ Primer, Fifth Edition

9.4. How a vector Grows

To support fast random access, vector elements are stored contiguously—each element is adjacent to the previous element. Ordinarily, we should not care about how a library type is implemented; all we should care about is how to use it. However, in the case of vectors and strings, part of the implementation leaks into its interface.

Given that elements are contiguous, and that the size of the container is flexible, consider what must happen when we add an element to a vector or a string: If there is no room for the new element, the container can’t just add an element somewhere else in memory—the elements must be contiguous. ...