Question

In modern C++, the idiom for iterating a sequential collection like string or vector when you only need the value of each element is short and elegant:

for (auto x: xs)

When you also need the index, it’s a little less elegant:

for (size_t i = 0; i != xs.size() ++i)

… unless there’s some recent development I haven’t yet caught up with. Does C++11 have a preferred way of doing the latter, or is the above still as good as it gets?

Answer 1

The preferred and idiomatic way is the simple for loop.

Alternative methods include using an integer range:

template<typename C>
auto container_index(C const& container) -> decltype(boost::irange(0, container.size())) {
  return boost::irange(0, container.size());
}

for(auto x : container_index(xs))

Or an iterating function:

template<typename F>
void index_iterate(std::size_t size, F func) {
  for(std::size_t i = 0; i != size; ++i) {
    func(i);
  }
}

index_iterate(container.size(), [&](std::size_t i){ /* ... */ });

Just go with the simple loop whenever possible though. It’s superior in my opinion.