What happens to a std::vector when an element is added?

When adding an element to std::vector (for example, using push_back), the following steps occur:

1. Capacity check: The vector checks if the currently allocated memory (capacity) is sufficient to hold the new element.
2. Expansion if needed:
   • If size() < capacity(), the new element is simply placed in the next free spot, and size() increases by 1.
   • If size() == capacity(), the vector must increase its capacity. This is usually done by allocating a new, larger block of memory. The new block size is typically several times larger than the current one (e.g., doubled).
3. Copying/moving elements (during expansion): All existing elements from the old memory block are copied (or moved, if the element type supports move semantics and a suitable constructor/operator is available) to the new memory block.
4. Placing the new element: The new element is added at the end of the new memory block.
5. Releasing old memory: The old memory block is freed.
6. Updating pointers/sizes: Internal pointers (pointing to the start and end of data, as well as to the end of allocated memory) are updated, and the size() value is increased.

This operation can be costly because of potential reallocation of memory, copying/moving elements, and freeing old memory. It can also invalidate iterators and references pointing to elements in the vector before the addition.