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  1.     template<class T, class R = void>  
  2.     struct enable_if_type
  3.     {
  4.         typedef R type;
  5.     };
  6.  
  7.     template<class E, class Enable = void>
  8.     struct GetFloatType
  9.     {
  10.         typedef E type;
  11.     };
  12.  
  13.     template<class E>
  14.     struct GetFloatType<E, typename enable_if_type<typename E::Scalar>::type>
  15.     {
  16.         typedef typename E::Scalar type;
  17.     };
  18.  
  19.     template<class E>
  20.     struct GetFloatType<E, typename enable_if_type<typename E::Inner>::type>
  21.     {
  22.         typedef typename E::Inner type;
  23.     };
  24.  
  25.  #include <typeinfo>
  26.  #include <iostream>
  27.     template <class Elemtype, class Floattype = typename GetFloatType<Elemtype>::type>
  28.     class ExponentialSmoother
  29.     {
  30.         public:
  31.         void print()
  32.         {
  33.             std::cout << "Elem: " << typeid(Elemtype).name()
  34.                 << ", Float: " << typeid(Floattype).name() << "\n";
  35.         }
  36.     };
  37.  
  38.     template<typename T>
  39.     class Vector
  40.     {
  41.         public:
  42.         typedef T Scalar;
  43.     };
  44.  
  45.     template<typename T>
  46.     class Vector2
  47.     {
  48.         public:
  49.         typedef T Inner;
  50.     };
  51.  
  52.     class Vec : public Vector<double>, public Vector2<float>
  53.     {
  54.  
  55.     };
  56.  
  57. int main()
  58. {
  59.     ExponentialSmoother<Vector<float> >().print();
  60.     ExponentialSmoother<Vector2<float> >().print();
  61.     ExponentialSmoother<double>().print();
  62.     ExponentialSmoother<double,char>().print();
  63.     // The following is ambiguous: Vec::Inner or Vec::Scalar ?
  64.     //ExponentialSmoother<Vec>().print();
  65. }
Success #stdin #stdout 0.01s 2680KB
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stdin
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Standard input is empty
stdout
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Elem: 6VectorIfE, Float: f
Elem: 7Vector2IfE, Float: f
Elem: d, Float: d
Elem: d, Float: c
https://ideone.com/cXNBha
language:
C++ (gcc 8.3)
created:
11 years ago
visibility:
public

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