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  1. #include <iostream>
  2. #include <type_traits>
  3.  
  4. struct BrokenDataWrapperByMoveOrCopy;
  5.  
  6. struct Data {
  7.   Data()                 { std::cout << "  constructor\n";}
  8.   Data(const Data& data) { std::cout << "  copy constructor\n";} 
  9.   Data(Data&& data)      { std::cout << "  move constructor\n";}
  10.   explicit Data(int i) { std::cout << " explicitly constructed from int\n";}
  11.   private:
  12.   explicit Data(void* p)  { std::cout << " only my friend knows how to construct me from a void*\n"; }
  13.   friend BrokenDataWrapperByMoveOrCopy;
  14. };
  15.  
  16. struct DataWrapperByMoveOrCopy {
  17.   Data data_;
  18.   template<typename T, 
  19.   	typename = typename std::enable_if<
  20.   		std::is_same<typename std::decay<T>::type, Data>::value
  21.   	>::type
  22.   >
  23.   DataWrapperByMoveOrCopy(T&& data) : data_(std::forward<T>(data)) { }
  24. };
  25.  
  26. struct BrokenDataWrapperByMoveOrCopy {
  27.   Data data_;
  28.   template<typename T>
  29.   BrokenDataWrapperByMoveOrCopy(T&& data) : data_(std::forward<T>(data)) { }
  30. };
  31.  
  32. int main() {
  33.  
  34.   std::cout << "1. DataWrapperByMoveOrCopy:\n";  
  35.   Data d1;
  36.   DataWrapperByMoveOrCopy a1(std::move(d1));
  37.  
  38.   std::cout << "2. BrokenDataWrapperByMoveOrCopy:\n";  
  39.   int d2;  //oops is an int rather than a Data
  40.   BrokenDataWrapperByMoveOrCopy a2(std::move(d2));  //this compiles, ouch
  41.  
  42.   std::cout << "3. DataWrapperByMoveOrCopy:\n";  
  43.   int d3;  //oops is an int rather than a Data
  44.   //DataWrapperByMoveOrCopy a3(std::move(d3));  //this failes as its supposed to
  45.  
  46.   std::cout << "4. BrokenDataWrapperByMoveOrCopy:\n";  
  47.   int* p = nullptr;
  48.   BrokenDataWrapperByMoveOrCopy a4(p);  //this compiles, this is terrible
  49.  
  50. }
Success #stdin #stdout 0s 3340KB
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stdin
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Standard input is empty
stdout
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1. DataWrapperByMoveOrCopy:
  constructor
  move constructor
2. BrokenDataWrapperByMoveOrCopy:
 explicitly constructed from int
3. DataWrapperByMoveOrCopy:
4. BrokenDataWrapperByMoveOrCopy:
 only my friend knows how to construct me from a void*
https://ideone.com/yb4e3Z
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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