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  1. #include <iostream>
  2. #include <type_traits>
  3.  
  4.  
  5. // Wrapper #######################################
  6.  
  7. template<class T>
  8. struct Wrapper {typedef T WrappedType;};
  9.  
  10. template<class T, class Enable=void>
  11. struct IsWrapper: std::false_type {};
  12.  
  13. template<class T>
  14. struct IsWrapper<Wrapper<T> >: std::true_type {};
  15.  
  16.  
  17. // WrapperTraits #######################################
  18.  
  19. template<
  20.   class T,
  21.   bool HasWrapper=
  22.     IsWrapper<T>::value && IsWrapper<typename T::WrappedType>::value>
  23. struct GetRootType {
  24.   static_assert(IsWrapper<T>::value,"T is not a wrapper type");
  25.  
  26.   typedef typename T::WrappedType RootType;
  27. };
  28.  
  29. template<class T>
  30. struct GetRootType<T,true> {
  31.   typedef typename GetRootType<typename T::WrappedType>::RootType RootType;
  32. };
  33.  
  34. template<class T>
  35. struct WrapperTraits {
  36.   typedef typename GetRootType<T>::RootType RootType;
  37. };
  38.  
  39.  
  40. // Test function #######################################
  41.  
  42. void f(int) {
  43.   std::cout<<"int"<<std::endl;
  44. }
  45.  
  46. // #define ROOT_TYPE_ACCESSOR WrapperTraits  // <-- Causes compilation error.
  47. #define ROOT_TYPE_ACCESSOR GetRootType    // <-- Compiles without error.
  48.  
  49. template<class T>
  50. auto f(T) -> 
  51.   typename std::enable_if<
  52.     std::is_same<int,typename ROOT_TYPE_ACCESSOR<T>::RootType>::value
  53.   >::type
  54. {
  55.   typedef typename ROOT_TYPE_ACCESSOR<T>::RootType RootType;
  56.  
  57.   std::cout<<"Wrapper<...<int>...>"<<std::endl;
  58.   f(RootType());
  59. }
  60.  
  61.  
  62. int main() {
  63.   f(Wrapper<int>());  
  64.   return 0; 
  65. }
Success #stdin #stdout 0s 3096KB
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stdin
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Standard input is empty
stdout
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Wrapper<...<int>...>
int
https://ideone.com/yXn4Oo
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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