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  1. #include <iostream>
  2. #include <type_traits>
  3.  
  4. struct Data {
  5.   Data()                 { std::cout << "  constructor\n";}
  6.   Data(const Data& data) { std::cout << "  copy constructor\n";} 
  7.   Data(Data&& data)      { std::cout << "  move constructor\n";}
  8. };
  9.  
  10. struct DataWrapperWithMove {
  11.   Data data_;
  12.   DataWrapperWithMove(Data&& data) : data_(std::move(data)) { }
  13. };
  14.  
  15. struct DataWrapperByValue {
  16.   Data data_;
  17.   DataWrapperByValue(Data data) : data_(std::move(data)) { }
  18. };
  19.  
  20. struct DataWrapperByMoveOrCopy {
  21.   Data data_;
  22.   template<typename T, 
  23.   	typename = typename std::enable_if<
  24.   		std::is_same<typename std::decay<T>::type, Data>::value
  25.   	>::type
  26.   >
  27.   DataWrapperByMoveOrCopy(T&& data) : data_(std::forward<T>(data)) { }
  28. };
  29.  
  30. struct Foo {
  31.   Foo()                 { std::cout << "  constructor\n";}
  32.   Foo(const Foo& data) { std::cout << "  copy constructor\n";} 
  33.   Foo(Foo&& data)      { std::cout << "  move constructor\n";}
  34. };
  35.  
  36. struct Bar {
  37.   Bar()                 { std::cout << "  constructor\n";}
  38.   Bar(const Bar& data) { std::cout << "  copy constructor\n";} 
  39.   Bar(Bar&& data)      { std::cout << "  move constructor\n";}
  40. };
  41.  
  42. struct Baz {
  43.   Baz()                 { std::cout << "  constructor\n";}
  44.   Baz(const Baz& data) { std::cout << "  copy constructor\n";} 
  45.   Baz(Baz&& data)      { std::cout << "  move constructor\n";}
  46. };
  47.  
  48. struct CompositeWrapperByMoveOrCopy {
  49.   Data data_;
  50.   Foo foo_;
  51.   Bar bar_;
  52.   Baz baz_;
  53.   template<typename T, typename U, typename V, typename W, 
  54.   	typename = typename std::enable_if<
  55.   		std::is_same<typename std::decay<T>::type, Data>::value &&
  56.   		std::is_same<typename std::decay<U>::type, Foo>::value &&
  57.   		std::is_same<typename std::decay<V>::type, Bar>::value &&
  58.   		std::is_same<typename std::decay<W>::type, Baz>::value
  59.   	>::type
  60.   >
  61.   CompositeWrapperByMoveOrCopy(T&& data, U&& foo, V&& bar, W&& baz) : 
  62.   data_{ std::forward<T>(data) },
  63.   foo_{ std::forward<U>(foo) },
  64.   bar_{ std::forward<V>(bar) },
  65.   baz_{ std::forward<W>(baz) } { }
  66. };
  67.  
  68. Data function_returning_data() {
  69.   return Data();
  70. }
  71.  
  72. int main() {
  73.   std::cout << "1. DataWrapperWithMove:\n"; 
  74.   Data d1;
  75.   DataWrapperWithMove a1(std::move(d1));
  76.  
  77.   std::cout << "2. DataWrapperByValue:\n";  
  78.   Data d2;
  79.   DataWrapperByValue a2(std::move(d2));
  80.  
  81.   std::cout << "3. DataWrapperByMoveOrCopy:\n";  
  82.   Data d3;
  83.   DataWrapperByMoveOrCopy a3(std::move(d3));
  84.  
  85.   std::cout << "4. DataWrapperByMoveOrCopy with l-value:\n"; 
  86.   Data d4;
  87.   DataWrapperByMoveOrCopy a4(d4);
  88.  
  89.   std::cout << "5. RVO:\n";
  90.   DataWrapperByValue a5(function_returning_data());
  91.  
  92.   std::cout << "6. CompositeWrapperByMoveOrCopy with l-value:\n"; 
  93.   Data d6;
  94.   Foo f;
  95.   Bar b;
  96.   Baz z;
  97.   int i;
  98.   CompositeWrapperByMoveOrCopy a6(d6, std::move(f), b, std::move(z)); //works
  99.   //CompositeWrapperByMoveOrCopy a7(d6, std::move(f), z, std::move(b)); //SFINAE kills it
  100.   //CompositeWrapperByMoveOrCopy a8(d6, std::move(f), b, i); //SFINAE kills it
  101. }
Success #stdin #stdout 0s 3296KB
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Standard input is empty
stdout
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1. DataWrapperWithMove:
  constructor
  move constructor
2. DataWrapperByValue:
  constructor
  move constructor
  move constructor
3. DataWrapperByMoveOrCopy:
  constructor
  move constructor
4. DataWrapperByMoveOrCopy with l-value:
  constructor
  copy constructor
5. RVO:
  constructor
  move constructor
6. CompositeWrapperByMoveOrCopy with l-value:
  constructor
  constructor
  constructor
  constructor
  copy constructor
  move constructor
  copy constructor
  move constructor
https://ideone.com/UsltRA
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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