member pointer to pointer constexpr conversion

ptr_from_mptr.cpp

#include <type_traits>
#include <utility>


namespace Impl {

// <e>   <e>      <e> <e>
// const            
//       volatile   
//                 & 
//                &&  
//                    noexcept [only >= c++17]
//  2   * 2      * 3 * 2                       = 24 template spec

template<class...>
struct args_impl;

template<class T>
struct mem_ptr_impl;

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...)> { 
    using this_t = Class;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...);
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) const> { 
    using this_t = const Class;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...);
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) volatile> { 
    using this_t = volatile Class;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...);
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) const volatile> { 
    using this_t = const volatile Class;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...);
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) &> { 
    using this_t = Class &;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...);
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) const &> { 
    using this_t = const Class &;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...);
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) volatile &> { 
    using this_t = volatile Class &;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...);
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) const volatile &> { 
    using this_t = const volatile Class &;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...);
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) &&> { 
    using this_t = Class &&;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...);
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) const &&> { 
    using this_t = const Class &&;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...);
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) volatile &&> { 
    using this_t = volatile Class &&;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...);
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) const volatile &&> { 
    using this_t = const volatile Class &&;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...);
};


template<typename T, typename U = void>
constexpr bool is_constexpr_v = false;


template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) noexcept> { 
    using this_t = Class;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...) noexcept;
    using is_noexcept = std::true_type;
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) const noexcept> { 
    using this_t = const Class;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...) noexcept;
    using is_noexcept = std::true_type;
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) volatile noexcept> { 
    using this_t = volatile Class;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...) noexcept;
    using is_noexcept = std::true_type;
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) const volatile noexcept> { 
    using this_t = const volatile Class;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...) noexcept;
    using is_noexcept = std::true_type;
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) & noexcept> { 
    using this_t = Class &;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...) noexcept;
    using is_noexcept = std::true_type;
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) const & noexcept> { 
    using this_t = const Class &;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...) noexcept;
    using is_noexcept = std::true_type;
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) volatile & noexcept> { 
    using this_t = volatile Class &;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...) noexcept;
    using is_noexcept = std::true_type;
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) const volatile & noexcept> { 
    using this_t = const volatile Class &;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...) noexcept;
    using is_noexcept = std::true_type;
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) && noexcept> { 
    using this_t = Class &&;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...) noexcept;
    using is_noexcept = std::true_type;
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) const && noexcept> { 
    using this_t = const Class &&;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...) noexcept;
    using is_noexcept = std::true_type;
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) volatile && noexcept> { 
    using this_t = volatile Class &&;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...) noexcept;
    using is_noexcept = std::true_type;
};

template<class Res, class Class, class ...Args>
struct mem_ptr_impl<Res (Class::*)(Args...) const volatile && noexcept> { 
    using this_t = const volatile Class &&;
    using args_t = args_impl<Args...>;
    using res_t = Res;
    using fun_ptr_t = res_t (*)(std::remove_reference_t<this_t>*, Args...) noexcept;
    using is_noexcept = std::true_type;
};

template<typename T>
constexpr bool is_constexpr_v<T, std::void_t<typename T::is_noexcept>> = typename T::is_noexcept{}();



template<class fun_ptr_t, class this_t, class res_t, class args_t, bool cexpr, auto N>
constexpr fun_ptr_t ptr_from_mptr_impl = [] {
    static_assert(!std::is_same_v<this_t, this_t>, "Not a member pointer");
    return nullptr;
} ();

template<class fun_ptr_t, class this_t, class res_t, class ... Args, bool cexpr, auto N>
constexpr fun_ptr_t ptr_from_mptr_impl<fun_ptr_t, this_t, res_t, args_impl<Args...>, cexpr, N> = 
+[](std::remove_reference_t<this_t>* this_, Args... args) noexcept(cexpr) -> res_t {
    return (std::forward<this_t>(*this_).*N)(std::forward<Args>(args)...);
};

}

template<auto N, typename mptr_impl = Impl::mem_ptr_impl<std::remove_reference_t<decltype(N)>>>
constexpr auto ptr_from_mptr = Impl::ptr_from_mptr_impl<typename mptr_impl::fun_ptr_t, 
                                                        typename mptr_impl::this_t, 
                                                        typename mptr_impl::res_t, 
                                                        typename mptr_impl::args_t, 
                                                        Impl::is_constexpr_v<mptr_impl>, N>;


#include <iostream>

struct A {
    void fun1() {
        std::cout << "fun1 called\n";
    }

    int fun2(float f) const {
        return f*2;
    }
};

int main() {

    A a;

    constexpr auto const_fun1 = ptr_from_mptr<&A::fun1>;
    constexpr auto const_fun2 = ptr_from_mptr<&A::fun2>;

    const_fun1(&a);

    std::cout << const_fun2(&a, 21) << "\n";
}