c++ - How to store template type in structure -

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#include<iostream> #include<typeinfo> using namespace std; template <class key, class value> struct st {         typedef key keytype;         typedef value valuetype; }; int main() {         st<int, int> st1;         if(typeid(st1.keytype) == typeid(int))                 cout<<"yes";         return 0; }

is there way of storing type key , value within structure? st1 structure stores key of type int , value of type int or template type within structure. want later use type comparison. following error.

 invalid use of ‘st<int, int>::keytype’   if(typeid(st1.keytype) == typeid(int))

i want store type initialized to stored within structure.

if can use c++11 or newer, can use decltype

    if ( typeid(decltype(st1)::keytype) == typeid(int) )             cout<<"yes";

or, better imho, std::is_same (evaluated compile-time)

    if ( std::is_same<decltype(st1)::keytype, int>::value )             cout<<"yes";

if can use c++17, last example can written as

    if constexpr ( std::is_same<decltype(st1)::keytype, int>::value )             cout<<"yes";

so cout << "yes" compiled or not according value of std::is_same.





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