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  1. #include <iostream>
  2. #include <functional>
  3. #include <string>
  4. #include <sstream>
  5.  
  6. struct Variant
  7. {
  8.     template<typename T>
  9.     explicit Variant(T x) { std::stringstream ss; ss << x; impl = ss.str(); }
  10.  
  11.     template<typename T>
  12.     friend T cast(Variant v) { std::stringstream ss(v.impl); T t; ss >> t; return t; }
  13.  
  14. private:
  15.     std::string impl;
  16. };
  17.  
  18. namespace details
  19. {
  20.     template<typename... Ts> struct type_seq {};
  21.  
  22.     template<unsigned... Ns> struct num_seq {};
  23.     template<unsigned I, unsigned... Ns> struct gen_num_seq : gen_num_seq<I - 1, I - 1, Ns...> {};
  24.     template<unsigned... Ns> struct gen_num_seq<0, Ns...> { typedef num_seq<Ns...> type; };
  25.  
  26.     template<typename F, unsigned... Is, typename... Ts>
  27.     Variant invoke2(F f, Variant* args, num_seq<Is...>, type_seq<Ts...>)
  28.     {
  29.         using namespace std; // workaround for a VC++ bug ('basic_string<...>' : undeclared identifier)
  30.         return Variant(f(cast<Ts>(args[Is])...));
  31.     }
  32.  
  33.     template<typename F, typename... Ts>
  34.     Variant invoke(F f, Variant* args, type_seq<Ts...> argTypes)
  35.     {
  36.         return invoke2(f, args, typename gen_num_seq<sizeof...(Ts)>::type(), argTypes);
  37.     }
  38.  
  39.     template<typename C, typename R, typename... A> auto get_arg_types_aux(R(C::*)(A...)) -> type_seq<A...>;
  40.     template<typename C, typename R, typename... A> auto get_arg_types_aux(R(C::*)(A...) const) -> type_seq<A...>;
  41.     template<typename R, typename... A> auto get_arg_types_aux(R(A...)) -> type_seq<A...>;
  42.     template<typename R, typename... A> auto get_arg_types_aux(R(*)(A...)) -> type_seq<A...>;
  43.     template<typename T> auto get_arg_types_aux(const T&) -> decltype(get_arg_types_aux(&T::operator()));
  44.     template<typename F> auto get_arg_types(F&& f) -> decltype(get_arg_types_aux(f)) { return decltype(get_arg_types_aux(f))(); }
  45. }
  46.  
  47. template<typename F>
  48. Variant invoke(F f, Variant* args)
  49. {
  50.     return details::invoke(f, args, details::get_arg_types(f));
  51. }
  52.  
  53. int main()
  54. {
  55.     auto test = [](int a, double b, std::string c) -> int
  56.     {
  57.         std::cout << "a=" << a << '\n';
  58.         std::cout << "b=" << b << '\n';
  59.         std::cout << "c=" << c << '\n';
  60.         return 42;
  61.     };
  62.  
  63.     Variant a[] = {Variant("20"), Variant("1.5"), Variant("x")};
  64.     std::cout << cast<int>(invoke(test, a)) << '\n';
  65. }
  66.  
Success #stdin #stdout 0s 3072KB
comments (?)
stdin
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Standard input is empty
stdout
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a=20
b=1.5
c=x
42
https://ideone.com/asMxsn
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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