How does the vector::size() returns size of the vector in constant time?

Question

I was going through this C++ Reference and found that using vector::size() returns the size of the vector in constant time. But, I wonder how one could get the size without actually traversing the vector.

Answer 1

It merely keeps track of the number of elements.

If that changes then that number is updated. For example, the number of elements increases by 1 when push_back or emplace_back is called. This is one of a number of reasons why std::vector is not intrinsically thread-safe.

As you can imagine, this makes the implementation of std::vector quite fiddly - the same can be said for other C++ standard library containers too - which is a good reason for not attempting to write container classes yourself.