c++ - Extracting function argument types as a parameter pack -

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this followup question "unpacking" tuple call matching function pointer, asked how provide values std::tuple arguments function in generic way. solution given there following:

template<int ...> struct seq { };  template<int n, int ...s> struct gens : gens<n-1, n-1, s...> { };  template<int ...s> struct gens<0, s...> {    typedef seq<s...> type; };  double foo(int x, float y, double z) {    return x + y + z; }  template <typename... args> struct save_it_for_later {    std::tuple<args...> params;    double (*func)(args...);     double delayed_dispatch()    {     return callfunc(typename gens<sizeof...(args)>::type());    }     template<int ...s>    double callfunc(seq<s...>)    {     return func(std::get<s>(params) ...);    } };  int main(void) {    std::tuple<int, float, double> t = std::make_tuple(1, 1.2, 5);    save_it_for_later<int,float, double> saved = {t, foo};    std::cout << saved.delayed_dispatch() << std::endl; }

my question whether there's way make alternate version of save_it_for_later takes foo template argument, don't have provide foo 's parameter types template argument (or bake return type save_it_for_later). like

int main(void) {    ...    save_it_for_later2<foo> saved = {t};    ... }

i'd equally fine sort of macro wrapping foo extract required types:

int main(void) {    ...    save_it_for_later<macro_using_decltype_or_somesuch(foo)> saved = {t};    ... }

this concern seems orthogonal enough original question warrant own ticket.

#include <tuple> #include <utility>  template <typename> struct save_it_for_later_t; template <typename result, typename... args> struct save_it_for_later_t<result (*)(args...)> {     std::tuple<args...>   params;     result              (*fun)(args...);     template <typename... params>     save_it_for_later_t(result (*fun)(args...), params&&... params)         : params(std::forward<params>(params)...)         , fun(fun) {     }     // ...  }; template <typename result, typename... args, typename... params> save_it_for_later_t<result(*)(args...)> save_it_for_later(result (*fun)(args...), params&&... params) {     return save_it_for_later_t<result(*)(args...)>(fun, std::forward<params>(params)...); }  double foo(float, float, double); int main() {     auto saved = save_it_for_later(foo, 1.2f, 3.4f, 5.6);     // ... }

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