c++ - Variadic template function unpack order -

i have code:

template<typename ...t> struct test {     void call(string str)     {         abc(get<t>(str)...);     }      template<typename u>     string get(string& inp)     {         string ret{ inp[0] };         inp.erase(0, 1);          cout << ret << endl; // first "a", next "b", next "c" - ok          return ret;     }      void abc(string a, string b, string c)     {         cout << << " " << b << " " << c << endl; // "b c a" - why?     } }; 

i'm calling this:

test<int, bool, float> test; test.call("abc"); 

and output b c a thought expect a b c. in get() function have correct order. why this? can't find rule order.

the order of evaluation of function arguments unspecified.

abc(get<t>(str)...); 

that same as:

abc(get<t1>(str), get<t2>(str), get<tn>(str)); 

you enforce evaluation order generating array store strings, dispatching array:

template <std::size_t n, std::size_t... idx> void call_helper(std::array<std::string, n> arr, std::index_sequence<idx...>) {     abc(std::get<idx>(arr)...); }  void call(string str) {     std::array<std::string,sizeof...(t)> arr { get<t>(str)... };      call_helper(arr, std::index_sequence_for<t...>{}); }