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  1. #include <iostream>
  2. #include <vector>
  3. #include <map>
  4.  
  5.     template<typename T>
  6.     struct MyFirstElement;
  7.     template<template<typename...> class T_Container, typename T_Type, typename... Ts>
  8.     struct MyFirstElement<T_Container<T_Type,Ts...>>{
  9.     	using Type = T_Type;
  10.     };
  11.  
  12.     //unit test
  13.     using MustBeInt = typename MyFirstElement<std::vector<int>>::Type;
  14.     using MustBeLong = typename MyFirstElement<std::map<long,float>>::Type;
  15.  
  16.     static_assert(std::is_same<MustBeInt,int>::value,"");
  17.     static_assert(std::is_same<MustBeLong,long>::value,""); 
  18.  
  19.     //this works too even though its not an STL container
  20.     template<typename T, typename U, typename Z>
  21.     struct MyCrazyContainer{};
  22.  
  23.     static_assert(std::is_same<typename MyFirstElement<MyCrazyContainer<bool,long,float>>::Type,bool>::value,"");
  24.  
  25. int main() {
  26. 	return 0;
  27. }
Success #stdin #stdout 0s 3336KB
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https://ideone.com/OxhSOc
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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