[c++17] replace boost optional with std::optional in zip_longest

[ryanhaining]

• update examples/zip_longest_examples.cpp to use structured bindings
• update README example to use structured bindings

[ryanhaining]

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

[xjjgjm]

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

[ryanhaining]

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

[xjjgjm]

#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;
        }
    }
}

[ryanhaining]

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

[xjjgjm]

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

[ryanhaining]

I can't change the library in a way that breaks existing users if they update to a newer version of the library.