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  1. #include <iostream>
  2. #include <algorithm>
  3. #include <vector>
  4. #include <iterator>
  5. #include <functional>
  6. using namespace std;
  7.  
  8. // the name is misleading and shows only what I
  9. // intended when starting to write this code. Now
  10. // a name like weird_thing would probably be more
  11. // appropriate.
  12. template <class T>
  13. class copyable_ref {
  14. public:
  15.   copyable_ref(T& ref) noexcept
  16.   : _ptr(std::addressof(ref)), _copied(false) {}
  17.   copyable_ref(T&&) = delete;
  18.   copyable_ref(const copyable_ref& x) noexcept
  19.   : _ptr (new int(*x._ptr)), _copied (true) {
  20.     cout << "copied " << x._ptr << ":" << *x._ptr << " to " << _ptr << endl;
  21.   }
  22.   ~copyable_ref() {
  23.     if (_copied) {
  24.       cout << "freed " <<  _ptr << ":" << *_ptr << endl;
  25.       delete _ptr;
  26.     }
  27.   }
  28.  
  29.   copyable_ref& operator=(const copyable_ref& x) noexcept {
  30.     cout << _ptr << ":" << *_ptr << " overwritten with " << x._ptr << ":" << *x._ptr << endl;
  31.     *_ptr = *x._ptr;
  32.    // _ptr = x._ptr;
  33.   }
  34.  
  35.   operator T& () const noexcept { return *_ptr; }
  36.   T& get() const noexcept { return *_ptr; }
  37.  
  38.   // unused by sort ... :/
  39.   void swap(copyable_ref & x) {
  40.     cout << "swap member" << endl;
  41.   }
  42.  
  43. private:
  44.   T* _ptr;
  45.   bool _copied;
  46. };
  47.  
  48. // I couldn't convince std::sort to use this.
  49. // That's why the copy constructor of the class
  50. // needs to be so weird.
  51. template<typename T>
  52. void swap(copyable_ref<T> & lhs, copyable_ref<T> & rhs) {
  53.   cout << "swap" << endl;
  54. }
  55.  
  56.  
  57. int main(int, char **) {
  58.  vector<int> input {1,2,3,4,5,6,7,8,9};
  59.  
  60.  // Vector with the references, reserving space isn't
  61.  // a performance thing, it's absolutely necessary to
  62.  // prevent the vector from copying elements.
  63.  vector<copyable_ref<int>> sorted;
  64.  sorted.reserve(input.size());
  65.  for (auto & e : input) {
  66.   if (e % 2 != 0) {
  67.     // emplace to prevent calling the copy constructor.
  68.     // We need this with references to the real items,
  69.     // and not to heap allocated copies of them.
  70.     sorted.emplace_back(e);
  71.   }
  72.  }
  73.  
  74.  copy(begin(sorted), end(sorted),
  75.       ostream_iterator<int>{cout, ", "});
  76.  cout << endl;
  77.  
  78.  // The actual sorting. This seems to contain code like
  79.  //   T t(a);
  80.  //   a = b;
  81.  //   b = t;
  82.  // instead of using swap. This is the reason for the
  83.  // strange copy constructor.
  84.  sort(begin(sorted), end(sorted),
  85.       greater<int>{});
  86.  
  87.  copy(begin(sorted), end(sorted),
  88.       ostream_iterator<int>{cout, ", "});
  89.  cout << endl;
  90.  
  91.  copy(begin(input), end(input),
  92.       ostream_iterator<int>{cout, ", "});
  93.  cout << endl;
  94.  return 0;
  95. }
Success #stdin #stdout 0s 3468KB
comments (?)
stdin 
Standard input is empty
stdout 
1, 3, 5, 7, 9, 
copied 0x9456a18:3 to 0x9456a88
0x9456a18:3 overwritten with 0x9456a10:1
0x9456a10:1 overwritten with 0x9456a88:3
freed 0x9456a88:3
copied 0x9456a20:5 to 0x9456a88
0x9456a20:5 overwritten with 0x9456a18:1
0x9456a18:1 overwritten with 0x9456a10:3
0x9456a10:3 overwritten with 0x9456a88:5
freed 0x9456a88:5
copied 0x9456a28:7 to 0x9456a88
0x9456a28:7 overwritten with 0x9456a20:1
0x9456a20:1 overwritten with 0x9456a18:3
0x9456a18:3 overwritten with 0x9456a10:5
0x9456a10:5 overwritten with 0x9456a88:7
freed 0x9456a88:7
copied 0x9456a30:9 to 0x9456a88
0x9456a30:9 overwritten with 0x9456a28:1
0x9456a28:1 overwritten with 0x9456a20:3
0x9456a20:3 overwritten with 0x9456a18:5
0x9456a18:5 overwritten with 0x9456a10:7
0x9456a10:7 overwritten with 0x9456a88:9
freed 0x9456a88:9
9, 7, 5, 3, 1, 
9, 2, 7, 4, 5, 6, 3, 8, 1,
https://ideone.com/HhrBGF
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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