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  1. #include <iostream>
  2. #include <type_traits>
  3. using namespace std;
  4.  
  5. namespace estd {
  6.  
  7. template<class T, class U = T, std::enable_if_t<
  8.         std::is_move_constructible<std::remove_reference_t<T>>::value && 
  9.         //std::is_assignable<std::remove_reference_t<T>, std::remove_reference_t<U>&&>::value && 
  10.         !std::is_lvalue_reference<U>::value,
  11.     int> = 0    
  12. >
  13. T exchange(T& obj, U&& new_value) noexcept(
  14.     std::is_nothrow_move_constructible<std::remove_reference_t<T>>::value &&
  15.     std::is_nothrow_assignable<std::remove_reference_t<T>, std::remove_reference_t<U>&&>::value
  16. )
  17. {
  18.     T old_value {std::move(obj)};
  19.     obj = std::move(new_value);
  20.     return old_value;
  21. }
  22.  
  23. } // namespace estd
  24. void do_stuff() noexcept
  25. { /*...*/ }
  26.  
  27. class Sample
  28. {
  29.     std::string mBody;
  30. public:
  31.     Sample(const std::string& body = ""): mBody {body} {}
  32.  
  33.     Sample(const Sample& s): mBody {s.mBody} {
  34.     	printf("%s\n", __PRETTY_FUNCTION__); // noexcept
  35.     }
  36.  
  37.     Sample& operator=(const Sample& s) { 
  38.     	mBody = s.mBody; 
  39.     	printf("%s\n", __PRETTY_FUNCTION__); // noexcept
  40.     	return *this;
  41.     }
  42.  
  43.     Sample(Sample&& dying) noexcept(
  44.         noexcept(do_stuff()) && 
  45.         noexcept(estd::exchange(dying.mBody, {}))
  46.     ):
  47.         mBody {estd::exchange(dying.mBody, {})}
  48.     {
  49.         do_stuff(); // noexcept
  50.     	printf("%s\n", __PRETTY_FUNCTION__); // noexcept
  51.     }
  52.  
  53.     Sample& operator=(Sample&& dying) noexcept(
  54.         noexcept(do_stuff()) && 
  55.         noexcept(estd::exchange(dying.mBody, {}))
  56.     )
  57.     {
  58.     	mBody = estd::exchange(dying.mBody, {});
  59.     	do_stuff();
  60.     	printf("%s\n", __PRETTY_FUNCTION__); // noexcept
  61.     	return *this;
  62.     }
  63.  
  64.     std::string body() const noexcept {return mBody;}
  65. };
  66.  
  67. int main()
  68. {
  69.     std::cout << std::boolalpha;
  70.     Sample rval{"wow such string very content"};
  71.     Sample dummy;
  72.     std::cout << noexcept( Sample(std::move(rval)) ) << std::endl; // prints true
  73.     std::cout << noexcept( dummy = std::move(rval) ) << std::endl; // prints true
  74.  
  75.     // The rest only to show that move semantics actually work
  76.     Sample f (std::move(rval));             // Calls move ctor
  77.     std::cout << rval.body() << std::endl;  // prints empty line, empty string moved in rval
  78.     std::cout << f.body() << std::endl;     // prints wow such string very content
  79.  
  80.     // estd::exchange(f, rval); // Fails to compile bc rval is an lvalue reference, template disabled
  81.     std::cout << (estd::exchange(f, std::move(rval))).body() << std::endl; // Ok, calls move ctor and move assignment (in estd::exchange) and prints wow such string very content
  82.     std::cout << f.body() << std::endl; // empty line, rval (empty) moved in f
  83.     std::cout << "end" << std::endl;
  84. }
Success #stdin #stdout 0s 4468KB
comments (?)
stdin
copy 
Standard input is empty
stdout
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true
true
Sample::Sample(Sample&&)

wow such string very content
Sample::Sample(Sample&&)
Sample& Sample::operator=(Sample&&)
wow such string very content

end
https://ideone.com/Mhposz
language:
C++14 (gcc 8.3)
created:
6 years ago
visibility:
public

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