How to project std::tuple in c++20 constrained algorithms

Question

Suppose we have a array of std::pairs:

using Particle = std::pair<std::string, double>;
Particle particles[] {{"Electron", 0.511}, {"Muon", 105.66}, {"Tau", 1776.86}};

We can using C++20 ranges algorithms with different projection function to sort them according to different data member:

ranges::sort(particles, {}, &Particle::first);
ranges::sort(particles, {}, &Particle::second);

This seems very clean. But when I move Particle's data type to std::tuple:

using Particle = std::tuple<std::string, double>;

I can't using same projection metric anymore since std::tuple have no first or second member. Alternatively, just pass lambda and it's will work fine:

ranges::sort(particles, {}, [](const auto& t) { return std::get<0>(t); });
ranges::sort(particles, {}, [](const auto& t) { return std::get<1>(t); });

But is there neater project way to do that like this?

ranges::sort(particles, {}, &std::get<0>);

May I ask why you are moving towards a tuple? I am just wondering if perhaps we could do even better with a small `struct` (with useful names on fields) — AndyG, Nov 24 '20 at 20:13

score 3 · Accepted Answer · answered Nov 23 '20 at 07:13

You can provide your own functor to do this

namespace my
{
    template<size_t n>
    struct get_t
    {
        template<typename T>
        decltype(auto) operator()(T&& t) const
        {
            using std::get;
            return get<n>(std::forward<T>(t));
        }
    };

    template<size_t n>
    inline constexpr get_t<n> get;
}

ranges::sort(particles, {}, my::get<0>);

How to project std::tuple in c++20 constrained algorithms

1 Answers1