[Boost-users] Converting pointer-to-member to member index using Boost.PFR

6 Sep 2025

      Consider the following struct:

    struct Particle
    {
        float scale;
        float opacity;
        float rotation;
    };

I want the following to compile:

    static_assert(indexOf<&Particle::scale>() == 0);
    static_assert(indexOf<&Particle::opacity>() == 1);
    static_assert(indexOf<&Particle::rotation>() == 2);

I have managed to implement a working version using Boost.PFR:

    template <auto PM>
    consteval int indexOf()
    {
        using ClassType = typename PMTraits<decltype(PM)>::ClassType;
        constexpr ClassType obj{};

        std::size_t result = -1;

        [&]<auto... Is>(std::index_sequence<Is...>)
        {
            (...,
                (
                    (    static_cast<const void*>(&(boost::pfr::get<Is>(obj))) 
                    == static_cast<const void*>(&(obj.*PM))                  ) 
                    ? (result = Is) : 0
                )
            );    
        }(std::make_index_sequence<boost::pfr::tuple_size_v<ClassType>>{});

        return result;
    }

But I am displeased of the fact that I do need to create an object of type `ClassType` in order to make `obj.*PM` work.

Is there a better way of doing this that (1) does not require the creation of an object at compile-time and/or (2) is more efficient to compile?

[Boost-users] Converting pointer-to-member to member index using Boost.PFR

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