Unpacking containers with variable-size at compile-time, without needing two duplicate functions to do so.

unpack_constexpr.c++

#include <cstddef>

#include <span>


constexpr std::size_t populate(std::span<int> data) {
    std::size_t w = 0;
    for (; w < 1000; ++w) {
        // this should maybe be a bounds-check instead, but in theory
        // this should only be called on containers that are exactly
        // the right size...
        if (not data.empty()) data[w] = w;
    }
    return w;
}

#include <array>
#include <functional>

template <std::size_t func(std::span<int>)>
constexpr std::array<int, func({})> unpack() {
    std::array<int, func({})> data;
    func(data);
    return data;
}

constexpr auto CONSTANT = unpack<populate>();

template <int X, int Y>
constexpr std::size_t paramed(std::span<int> data) {
    const std::size_t Z = X * Y;
    std::size_t c = 0;
    for (; c < Z; ++c) {
        if (not data.empty()) data[c] = X * Y - c;
    }
    return c;
}

constexpr auto CLASS_CONSTANT = unpack<paramed<19, 12>>();


int main() {
    for (auto c : CONSTANT) {}
    for (auto c : CLASS_CONSTANT) {}
}