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,                                                             
     ^

A type with an `explicit` move constructor does not satisfy `MoveConstructible`. — T.C., Jun 16 '15 at 20:16
Though I see your initial issue is solved, there is something you should be aware of. The requirement for push_heap is only MoveConstructible/Assignable, yes. But you are also doing an emplace_back on the vector. vector has a more complicated requirement: you either need a noexcept move constructor/move assignment, or a copy constructor. Your example works because the default generated move functions are no except (in in this case); if you write it yourself you'll need to explicitly use noexcept (note that unique_ptr satifies this requirement). — Nir Friedman, Jun 17 '15 at 02:30
Thanks @NirFriedman. I was aware of that issue, but thanks for pointing it out. — Arindam, Jun 18 '15 at 20:20

score 3 · Accepted Answer · answered Jun 16 '15 at 20:12

3

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.

answered Jun 16 '15 at 20:12

Howard Hinnant

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Thank you. Yes, removing `explicit` gets it working again. – Arindam Jun 16 '15 at 20:54

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

Sample Code

Errors

1 Answers1