Question

In a C++ programming book I saw the following for a std::list iterator:

for (iterator = list.start(); iterator != list.end(); iterator++)

Isn’t it inefficient to call list.end() all the time? Would it be better to save the end to another variable or will the C++ compiler (i. e. g++) take care of this automatically?

Answer 1

The call to std::list<T>::end() is unlikely to be a big efficiency issue and probably just reads a single value. However, you’d give the compiler a hint that it isn’t meant to change by storing it a variable. For other containers a computation may be involved in addition to reading a base address which is a bit more involved. Still nothing dramatic but possibly worth avoiding.

Note, however, that it may also change the semantic of the loop: If the body of the loop may append elements, the former end may move. Interestingly, I don’t find any specific requirements in the standard stating whether std::list<T>::end() may change when inserting elements into the container (I can imagine implementations where it does change as well as some where it doesn’t; most likely it doesn’t change, though). If you want to get guaranteed behavior when also modifying the list, you might very well call list.end() in every iteration.

BTW, there is a bigger performance concern I’d have about using iterator++ instead of ++iterator, especially this is really what the author used in the book. Still, this is a micro optimization like storing the result of list.end() but one cheap to do.