No matter how useful iterators are, and what generic interface they represent, iterators can easily be misused, just as pointers. When dealing with pointers, code must be written in a way that it never dereferences them when they point to invalid memory locations. Same applies to iterators, but there are a lot of rules that state when an iterator is valid and when it is invalidated. Those can easily be learned by studying the STL documentation a bit, but it will still always be possible to write buggy code.

In the best case, such buggy code blows up in front of the developer while it is being tested, and not on the client's machine. However, in many cases, the code just silently seems to work, although it dereferences dangling pointers, iterators, and so on. In such cases, we want to be eagerly alarmed if we produce code showing undefined behavior.

Fortunately, there's help! The GNU STL implementation has a debug mode, and the GNU C++ compiler as well as the LLVM clang C++ compiler both support additional libraries that can be used to produce extra-sensitive and verbose binaries for us, which immediately blow up on a large variety of bugs. This is easy to use and super useful, as we will demonstrate in this section. The Microsoft Visual C++ standard library also provides a possibility to activate additional checks.