El 06/09/2025 a las 1:50, mail--- via Boost-users escribió:

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? _______________________________________________

Something like this (fails with Clang and I haven't tried with VS): https://godbolt.org/z/1K36Yfc58   #include <algorithm>   #include <array>   #include <boost/mp11/algorithm.hpp>   #include <boost/pfr/core_name.hpp>   #include <boost/type_index/ctti_type_index.hpp>   template<auto pm> struct pm_class_impl;   template<class C, typename T, T C::*pm>   struct pm_class_impl<pm>   {     using type = C;   };   template<auto pm> using pm_class = typename pm_class_impl<pm>::type;   template<auto x> struct pm_name_helper{};   template<auto pm>   consteval int index_of()   {     using C = pm_class<pm>;     constexpr std::size_t N = boost::pfr::tuple_size_v<C>;     std::array<std::string_view, N> names; boost::mp11::mp_for_each<boost::mp11::mp_iota_c<N>>([&](auto I) {       names[I] = boost::pfr::get_name<I,C>();     });     std::string_view mangled_pm_name{ boost::typeindex::ctti_type_index::type_id<pm_name_helper<pm>>().raw_name()     };     std::string_view pm_name{       mangled_pm_name.begin() + mangled_pm_name.find_last_of(':') + 1,       mangled_pm_name.begin() + mangled_pm_name.find_last_of('>')     };     return std::find(names.begin(), names.end(), pm_name) - names.begin();   }   struct Particle   {     float scale;     float opacity;     float rotation;   };   int main()   {     static_assert(index_of<&Particle::scale>() == 0);     static_assert(index_of<&Particle::opacity>() == 1);     static_assert(index_of<&Particle::rotation>() == 2);   } Joaquín M López Muñoz