fork download
copy 
  1. #include <iostream>
  2. #include <vector>
  3. #include <type_traits> //needed for enable_if?
  4. using namespace std;
  5.  
  6. template<class T> struct supports_iteration
  7. { 
  8. private:
  9. 	typedef char yes[1];
  10. 	typedef char no[2];
  11. 	template <class C> static yes& foo(typename C::iterator*);
  12. 	template <class C> static no& foo(...);
  13. public:
  14. 	static constexpr bool value = sizeof(foo<T>(0)) == sizeof(yes);
  15. };
  16.  
  17.  
  18. template<template <class, class> class C, class T, class A>
  19. void DoSomething(C<T,A>& val)
  20. {
  21. 	T* pT;
  22. 	cout << "did something!\n";
  23. }
  24.  
  25. template<template <class, class> class C, class T, class A>
  26. void DoSomethingSmartly(
  27. 	typename std::enable_if<
  28. 		supports_iteration<
  29. 			C<T,A>
  30. 		>::value
  31. 	>::type& val)
  32. {
  33. 	T* pT;
  34. 	cout << "did something smartly!\n";
  35. }
  36.  
  37. template<template <class, class> class C, class T, class A, 
  38.     typename = decltype(
  39.         declval<C<T,A>>().size()
  40.         ,void()
  41.     )
  42. >
  43. void DoSomethingReallySmartly(C<T,A>& val)
  44. {
  45. 	T* pT;
  46. 	cout << "did something really smartly!\n";
  47. }
  48.  
  49. int main() {
  50. 	// your code goes here
  51. 	vector<int> v{1,2,3,4,5};
  52. 	DoSomething(v);
  53. 	cout << "vector<int> supports_iteration? " << 
  54. 		boolalpha << supports_iteration<decltype(v)>::value << "!" << endl;
  55. 	//DoSomethingSmartly(v);// - fails!!
  56. 	//template argument deduction/substitution failed:
  57. 	//couldn't deduce template parameter ‘template<class, class> class C’  DoSomethingSmartly(v);
  58. 	DoSomethingReallySmartly(v);
  59. 	return 0;
  60. }
  61.  
Success #stdin #stdout 0s 16064KB
comments ()
stdin
copy 
Standard input is empty
stdout
copy 
did something!
vector<int> supports_iteration? true!
did something really smartly!
https://ideone.com/WKsZSN
decltype and declval SFINAE vs old sfinae
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

Share or Embed source code

Discover > Sphere Engine API

The brand new service which powers Ideone!

Discover > IDE Widget

Widget for compiling and running the source code in a web browser!