Can this for loop be done with the preprocessor?

Question

My code uses a library (FastLED) which uses a templated function:

#define NUM_WIDGETS 4

Library.function<1>();
Library.function<2>();
Library.function<3>();
Library.function<4>();

I can't put this in a normal for loop since the template argument needs to be computable at compile-time. Can I do this in the preprocessor? Any other suggestion? I'd like to be able to change NUM_WIDGETS conveniently without copy-pasting these lines.

Answer 1

You can do this using templates with the help of std::index_sequence:

constexpr size_t NUM_WIDGETS = 4;

template <size_t N>
void foo() {
    std::cout << N << '\n';
}

template <size_t... Ns>
void call_foo_helper(std::index_sequence<Ns...>) {
    (foo<Ns>(), ...);
}

template <size_t N>
void call_foo() {
    call_foo_helper(std::make_index_sequence<N>());
}

int main() {
    call_foo<NUM_WIDGETS>();
}

This uses C++17 fold expressions. If you don't have C++17 available, you could use a recursive template instead:

constexpr size_t NUM_WIDGETS = 4;

template <size_t N>
void foo() {
    std::cout << N << '\n';
}

template <size_t N>
void call_foo() {
    foo<N>();
    call_foo<N - 1>();
}

template <>
void call_foo<0>() { }

int main() {
    call_foo<NUM_WIDGETS>();
}