[c++17] replace boost optional with std::optional in zip_longest about cppitertools HOT 7 OPEN

std::optional doesn't support reference types. I can use a std::optional<std::reference_wrapper<T>> but that'll throw off uses with assignment

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Is it possible to replace boost::optional with C Pointers in zip_longest?

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I can't imagine how, do you have something in mind?

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#include <zip_longest.hpp>

#include <vector>
#include <string>
#include <iostream>

int main() {
    std::vector<int> ivec{1, 4, 9, 16, 25, 36};
    std::vector<std::string> svec{"hello", "good day", "goodbye"};

    // i: int*
    // s: std::string*
    for (auto&& [i, s] : iter::zip_longest(ivec, svec)) {
        if (i) {
            std::cout << *i << std::endl;
        }

        if (s) {
            std::cout << *s << std::endl;
        }
    }
}

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of course you could implement a zip_longest like that, but changes to the yielded type cannot break existing uses.

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Changing the type to a pointer doesn't change the existing usage, does it?

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I can't change the library in a way that breaks existing users if they update to a newer version of the library.

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