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  1. #include <assert.h>
  2.  
  3. // Helper to map any type to void, needed by SFINAE below
  4. template <typename T>
  5. struct void_type {
  6.     typedef void type;
  7. };
  8.  
  9. // Selects a nested typedef or a default type D (using a macro to reduce boilerplate):
  10. #define SELECT_NESTED_TYPE( TYPE )                                       \
  11. template <typename T, typename D, typename _ = void>                     \
  12. struct select_##TYPE{                                                    \
  13.     typedef D type;                                                      \
  14. };                                                                       \
  15. template <typename T, typename D>                                        \
  16. struct select_##TYPE<T, D, typename void_type<typename T::TYPE>::type> { \
  17.     typedef typename T::TYPE type;                                       \
  18. };                                                                      
  19.  
  20. SELECT_NESTED_TYPE( int_t );
  21. SELECT_NESTED_TYPE( float_t );
  22. //...
  23. #undef SELECT_NESTED_TYPE
  24.  
  25. // Use
  26. template <typename T>
  27. class TheTemplate {
  28. public:
  29.    typedef typename select_int_t<T,int>::type int_t;
  30.    typedef typename select_float_t<T,double>::type float_t;
  31.    //....
  32. };
  33.  
  34. // Test:
  35. template <typename T, typename U> struct same_type {
  36.    static const bool value = false;
  37. };
  38. template <typename T> struct same_type<T,T> {
  39.    static const bool value = true;
  40. };
  41. struct test1 {
  42. };
  43. struct test2 {
  44.    typedef long long int_t;
  45.    typedef float float_t;
  46. };
  47. int main() {
  48.    // test1 has the default typedefs
  49.    assert(( same_type< TheTemplate<test1>::int_t, int>::value ));
  50.    assert(( same_type< TheTemplate<test1>::float_t, double>::value ));
  51.    // test2 has the ones in the type
  52.    assert(( same_type< TheTemplate<test2>::int_t, long long>::value ));
  53.    assert(( same_type< TheTemplate<test2>::float_t, float>::value ));
  54. }
Success #stdin #stdout 0.02s 2720KB
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https://ideone.com/Gd4rQ
language:
C++ (gcc 8.3)
created:
13 years ago
visibility:
public

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