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  1. #include <functional>
  2. #include <iostream>
  3. #include <type_traits>
  4. #include <utility>
  5.  
  6. struct Vector3f64 {
  7.   double x;
  8.   double y;
  9.   double z;
  10. };
  11.  
  12. struct Vector3f32 {
  13.   float x;
  14.   float y;
  15.   float z;
  16. };
  17.  
  18. // I use this to select their element type in functions:
  19. template <typename T>
  20. using param_vector = std::conditional_t<std::is_same<std::remove_const_t<std::remove_reference_t<T>>, Vector3f64>::value, double, float>;
  21.  
  22. // This is the function I want to pull the return type from:
  23. template <typename T>
  24. T VectorVolume(const T x, const T y, const T z)
  25. {
  26.   return x * x + y * y + z * z;
  27. }
  28.  
  29. template<class R, class... ARGS>
  30. std::function<R(ARGS...)> make_func(R(*ptr)(ARGS...)) {
  31.   return std::function<R(ARGS...)>(ptr);
  32. }
  33.  
  34. // This function fails to compile:
  35. template <typename T>
  36. typename decltype(make_func(&VectorVolume<param_vector<T>>))::result_type func(const T& dir)
  37. {
  38.   return VectorVolume(dir.x, dir.y, dir.z);
  39. }
  40.  
  41. int main() {
  42.   const Vector3f64 foo{ 10.0, 10.0, 10.0 };
  43.  
  44.   std::cout << func(foo) << std::endl;
  45. }
  46.  
Success #stdin #stdout 0s 15232KB
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stdin 
Standard input is empty
stdout 
300
https://ideone.com/PU3oBV
language:
C++14 (gcc 8.3)
created:
5 years ago
visibility:
public

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