Accessing std::vector instances the fast or the safe way

The std::vector is probably the most widely used container in the STL, because it holds data just like an array, and adds a lot of comfort around that representation. However, wrong access to a vector can still be dangerous. If a vector contains 100 elements, and by accident our code tries to access an element at index 123, this is obviously bad. Such a program could just crash, which might be the best case, because that behavior would make it very obvious that there is a bug! If it does not crash, we might observe that the program just behaves strangely from time to time, which could lead to even more headaches than a crashing program. The experienced programmer might add some checks before any directly indexed vector access. Such checks do not increase the readability of the code, and many people do not know that...