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  1. #include <iostream>
  2. #include <type_traits>
  3. #include <vector>
  4. using namespace std;
  5.  
  6. // The forward_as template
  7. template<typename T, typename U>
  8. auto forward_as( U&& u ) -> typename std::conditional<
  9.         std::is_reference<T>::value, 
  10.         typename std::remove_reference<U>::type&, 
  11.         typename std::remove_reference<U>::type&&>::type
  12. {
  13.     return static_cast<decltype(forward_as<T, U>(u))>(u);
  14. }
  15.  
  16. // Structs for testing
  17. struct  X {};
  18. struct Y {
  19. 	vector<X> range;
  20. 	Y() : range(1) {}
  21. };
  22.  
  23. // bar will tell us the actual reference type of the forwarding reference
  24. template<typename T>
  25. void bar( T&& t ) {
  26. 	if (is_rvalue_reference<decltype(t)>::value)
  27. 		cout << "t is a r-value reference\n";
  28. 	else if (is_lvalue_reference<decltype(t)>::value)
  29. 		cout << "t is an l-value reference\n";
  30. 	else
  31. 		cout << "t is passed by value\n";
  32. }
  33.  
  34. // Elementwise perfect forwarding of a member range
  35. template<typename T>
  36. void foo( T&& t ) {
  37. 	for (auto& r : t.range)
  38. 		bar(forward_as<T>(r));
  39. }
  40.  
  41. // Naive and non-working attempt
  42. template<typename T>
  43. void naive_foo( T&& t ) {
  44. 	for (auto&& r : forward<T>(t).range)
  45. 		bar(r);
  46. }
  47.  
  48. int main() {
  49. 	// Pass by r-value reference.
  50. 	cout << "foo(Y()):\n\t";
  51. 	foo(Y());
  52. 	cout << "naive_foo(Y()):\n\t";
  53. 	naive_foo(Y());
  54.  
  55. 	// Pass by l-value reference.
  56. 	Y y;
  57. 	cout << "foo(y):\n\t";
  58. 	foo(y);
  59. 	cout << "naive_foo(y):\n\t";
  60. 	naive_foo(y);
  61. }
Success #stdin #stdout 0s 3428KB
comments (?)
stdin
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Standard input is empty
stdout
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foo(Y()):
	t is a r-value reference
naive_foo(Y()):
	t is an l-value reference
foo(y):
	t is an l-value reference
naive_foo(y):
	t is an l-value reference
https://ideone.com/e7ArjW
Sample code for https://c...content-available-to-author-only...e.com/questions/68651/elementwise-perfect-forwarding-of-a-member-range
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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