unused variable when using std::views::iota in C++20

Background

In traditional C++, we used to use an integer to make an iteration as below:

for (auto i = begin; i < end; ++i) {
    ...
}

However, this way looks verbose. We potentially need a simple and convenient way like in Python:

for i in range(begin, end):
    ...

std::views::iota

Fortunately, we can achieve it in C++20 as the below code:

#include <ranges>

for (auto i : std::views::iota(begin, end)) {
    ...
}

namespace std::views is an alias to namespace std::ranges::views with namespace views = ranges::views;. Therefore, std::views::iota is an abbreviation of std::ranges::views::iota.

Actually, their return type is std::ranges::iota_view so it’s also available to use std::ranges::iota_view(begin, end).

In conclusion, it’s suggested to use std::views::iota in everywhere for unification.

unused variable

There is an issue that the compiler would complained there was an unused variable i if it was only for iteration and it wasn’t used in the loop body.

In previous code, there was no such issue because the iterator i had been not only read but also written.

We might add attribute [[maybe_unused]] (C++17) to silence that warning:

for ([[maybe_unused]] int i : std::views::iota(0, 10)) {
    std::cout << "dummy" << std::endl;
}

References

• for-loop counter gives an unused-variable warning