Take by const T& for any non-trivial element type to skip the copy and document read-only intent. For built-in types like int, double, or pointers, by value is just as fast and reads cleaner. The classic mistake is for (auto x : v) over a vector<string>: it silently copies every element, which can dominate the loop's runtime on a large vector of strings. Make auto a default but pair it with & (for (const auto& x : v)) for the same reason.

A non-const reference (int&) lets you write through the loop variable, modifying the underlying container in place. This is shorter than the equivalent indexed loop and harder to get wrong: there is no i to forget to advance. Use it for transforms like normalisation, scaling, or in-place clamping. Be careful with auto& over a std::vector<bool> because it returns proxy references; that is one of the few cases where the indexed form is actually safer.

When you need the index alongside the value, the cleanest pre-C++20 form is a side counter that you increment inside the body. C++20's std::ranges::views::enumerate is even nicer but is not available on the GCC 9.2 / C++17 compiler used here. Range-for also works on plain C arrays of known size, no sizeof(arr)/sizeof(arr[0]) ritual needed, because the array decays gracefully into the begin/end machinery. The same loop form works on std::initializer_list, std::array, and any user type that implements begin() and end().