templates - C++ std::get equivalent with run-time arguments -


one cannot directly use std::get run-time arguments:

template<typename tuple> void get(size_t i, tuple const& t) {      using std::get;      std::cout<<get<i>(t)<<std::endl;       //error: 'i' not constant expression }  int main() {     std::tuple<int,double> t;     get(1,t); }

however, 1 can manually map run-time compile-time information:

template<typename tuple> void get_impl(size_t i, tuple const& t, typename std::tuple_size<tuple>::type) {}  template<size_t n, typename tuple, typename = std::enable_if_t<std::tuple_size<tuple>::value != n> > void get_impl(size_t i, tuple const& t, std::integral_constant<size_t, n>) {     if(i==n)     {         std::cout<<std::get<n>(t)<<std::endl;     }     else     {         get_impl(i, t, std::integral_constant<size_t, n+1>());     } }  template<typename tuple> void get(size_t i, tuple const& t) {     get_impl(i, t, std::integral_constant<size_t, 0>()); }

which seems rather conventient me (demo).

with this, 1 can apply arbitrary functions (which accept tuple members) tuple-element chosen @ run-time.

is there reason why or similar not in standard library? missing relevant here hinders proper application?


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