Question

Consider the following:

std::vector<int> vec(1); // vector has one element
std::fill(vec.begin(), vec.begin(), 42);
std::fill(vec.begin()+1, vec.end(), 43);
std::fill(vec.end(), vec.end(), 44);

Will all of the std::fill usages above result in defined behavior? Am I guaranteed that vec will remain unmodified? I’m inclined to think “yes”, but I want to make sure the standard allows such usage.

Answer 1

No, if doesn’t cause undefined behavior.

The standard defines empty iterator range in 24.1/7 and nowhere it says that supplying an empty range to std::fill algorithm causes undefined behavior.

This is actually what one would expect from a well-thought through implementation. With algorithms that handle emtpy range naturally, imposing the requirement to check for empty range on the caller would be a serious design error.