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  1. #include <set>
  2. #include <map>
  3. #include <iostream>
  4. #include <typeinfo>
  5.  
  6.     //// This is a SFINAE context enabler. If T is defined R is returned
  7.     template<class T, class R=void> struct enable_if_type
  8.     {
  9.        typedef R type;
  10.     };
  11.  
  12.     //// Default case is undefined as you want to get an error if you try to get a key_type from something that has none
  13.     template<class T, class Enable=void> struct key_type_of;
  14.  
  15.  
  16.     //// If T::key_type is a valid expression, extract it
  17.     template<class T>
  18.     struct key_type_of< T
  19.                       , typename enable_if_type< typename T::key_type>::type
  20.                       >
  21.     {
  22.       typedef typename T::key_type type;
  23.     };
  24.  
  25. int main()
  26. {
  27.   typedef key_type_of< std::set<int> >::type skey;
  28.   typedef key_type_of< std::map<std::string,std::string*> >::type mkey;
  29.  
  30.   std::cout << typeid(skey).name() << "\n";
  31.   std::cout << typeid(mkey).name() << "\n";
  32.  
  33. }
Success #stdin #stdout 0.01s 2680KB
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https://ideone.com/3PxJm
language:
C++ (gcc 8.3)
created:
11 years ago
visibility:
public

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