std::push_heap and std::pop_heap with MoveConstructible objects

Question

I want to maintain a heap where the payloads are MoveConstructible (because they hold a std::unique_ptr inside.) Although the documentation suggests that the object must be MoveAssignable and MoveConstructible , doing so throws an error both in GCC and Clang.

Sample Code

I don't use an std::unique_ptr yet, but just disable the corresponding copy operators):

#include <iostream>
#include <vector>
#include <algorithm>

struct Data {
  explicit Data(int value) : value(value) {}
  int value;

  bool operator<(const Data& d) const {
      return value > d.value;
  }

  /**
    * Commenting out the rest of the section for this struct
    * gets the code working by using copy operator/ctor.
    */ 
  explicit Data(const Data& d) = delete;
  Data& operator=(const Data& d) = delete;

  explicit Data(Data&& d) = default;
  Data& operator=(Data&& d) = default;    
};

struct MinHeap {
  void Push(Data&& d) {
    container_.emplace_back(std::move(d));
    std::push_heap(container_.begin(), container_.end());
  }

  Data Pop() {
    std::pop_heap(container_.begin(), container_.end());
    Data d = std::move(container_.back());
    container_.pop_back();
    return std::move(d);
  }

  size_t size() const { return container_.size(); }
  bool empty() const { return size() == 0; }

 private:
  std::vector<Data> container_;
};

int main() {
    MinHeap h;
    h.Push(Data(5));
    h.Push(Data(8));
    h.Push(Data(11));
    h.Push(Data(3));
    h.Push(Data(7));

    while (!h.empty()) {
        Data d = std::move(h.Pop());
        std::cout << d.value << std::endl;
    }
    return 0;
}

Any idea what I am doing wrong, and how I can get the heap working with move semantics ?

Errors

In file included from /usr/include/c++/4.9.2/bits/stl_pair.h:59:0,                                                                   
                 from /usr/include/c++/4.9.2/bits/stl_algobase.h:64,                                                                 
                 from /usr/include/c++/4.9.2/bits/char_traits.h:39,                                                                  
                 from /usr/include/c++/4.9.2/ios:40,                                                                                 
                 from /usr/include/c++/4.9.2/ostream:38,                                                                             
                 from /usr/include/c++/4.9.2/iostream:39,                                                                            
                 from main.cpp:1:                                                                                                    
/usr/include/c++/4.9.2/bits/stl_heap.h: In instantiation of 'void std::push_heap(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__norma
l_iterator<Data*, std::vector<Data> >]':                                                                                             
main.cpp:25:56:   required from here                                                                                                 
/usr/include/c++/4.9.2/bits/stl_heap.h:164:28: error: no matching function for call to 'Data::Data(std::remove_reference<Data&>::type
)'                                                                                                                                   
       _ValueType __value = _GLIBCXX_MOVE(*(__last - 1));                                                                            
                            ^                                                                                                        
In file included from /usr/include/c++/4.9.2/bits/stl_algo.h:61:0,                                                                   
                 from /usr/include/c++/4.9.2/algorithm:62,                                                                           
                 from main.cpp:3:                                                                                                    
/usr/include/c++/4.9.2/bits/stl_heap.h:167:45: error: no matching function for call to 'Data::Data(std::remove_reference<Data&>::type
)'                                                                                                                                   
          __gnu_cxx::__ops::__iter_less_val());                                                                                      
                                             ^                                                                                       
/usr/include/c++/4.9.2/bits/stl_heap.h:124:5: error:   initializing argument 4 of 'void std::__push_heap(_RandomAccessIterator, _Dist
ance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Data*, std::vector<Data> >; _Distance = lo
ng int; _Tp = Data; _Compare = __gnu_cxx::__ops::_Iter_less_val]'                                                                    
     __push_heap(_RandomAccessIterator __first,                                                                                      
     ^                                                                                                                               
In file included from /usr/include/c++/4.9.2/bits/stl_pair.h:59:0,                                                                   
                 from /usr/include/c++/4.9.2/bits/stl_algobase.h:64,                                                                 
                 from /usr/include/c++/4.9.2/bits/char_traits.h:39,                                                                  
                 from /usr/include/c++/4.9.2/ios:40,                                                                                 
                 from /usr/include/c++/4.9.2/ostream:38,                                                                             
                 from /usr/include/c++/4.9.2/iostream:39,                                                                            
                 from main.cpp:1:                                                                                                    
/usr/include/c++/4.9.2/bits/stl_heap.h: In instantiation of 'void std::__pop_heap(_RandomAccessIterator, _RandomAccessIterator, _Rand
omAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Data*, std::vector<Data> >; _Compare = __gnu_c
xx::__ops::_Iter_less_iter]':                                                                                                        
/usr/include/c++/4.9.2/bits/stl_heap.h:280:42:   required from 'void std::pop_heap(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__nor
mal_iterator<Data*, std::vector<Data> >]'                                                                                            
main.cpp:29:55:   required from here                                                                                                 
/usr/include/c++/4.9.2/bits/stl_heap.h:243:28: error: no matching function for call to 'Data::Data(std::remove_reference<Data&>::type
)'                                                                                                                                   
       _ValueType __value = _GLIBCXX_MOVE(*__result);                                                                                
                            ^                                                                                                        
In file included from /usr/include/c++/4.9.2/bits/stl_algo.h:61:0,                                                                   
                 from /usr/include/c++/4.9.2/algorithm:62,                                                                           
                 from main.cpp:3:                                                                                                    
/usr/include/c++/4.9.2/bits/stl_heap.h:247:35: error: no matching function for call to 'Data::Data(std::remove_reference<Data&>::type
)'                                                                                                                                   
     _GLIBCXX_MOVE(__value), __comp);                                                                                                
                                   ^                                                                                                 
/usr/include/c++/4.9.2/bits/stl_heap.h:207:5: error:   initializing argument 4 of 'void std::__adjust_heap(_RandomAccessIterator, _Di
stance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Data*, std::vector<Data> >; _Distance = 
long int; _Tp = Data; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'                                                                 
     __adjust_heap(_RandomAccessIterator __first, _Distance __holeIndex,                                                             
     ^                                                                                                                               

Answer 1

Never mark your copy constructor or move constructor as explicit . It isn't illegal. It is just unusual enough to be confusing. There is no benefit to it, and only downside.

The explicit on your move constructor is the cause of this error.