Creazione di una std :: unordered_map con una chiave std :: pair come chiave

Sto provando a creare una std :: unordered_map con una chiave std :: pair. Come puoi immaginare, questo mi richiederebbe di fornire esplicitamente una classe per generare un hash per una determinata chiave, oltre a un comparatore di uguaglianza per le chiavi. Ecco il mio codice finora:Creazione di una std :: unordered_map con una chiave std :: pair come chiave

#include <unordered_map> 
#include <memory> 
#include <utility> 

template <class T, typename U> 
struct PairHash{ 
    size_t operator()(const std::pair<T, U> &key){ 
     return std::hash<T>()(key.first)^std::hash<U>()(key.second); 
    } 
}; 

template <class T, typename U> 
struct PairEqual{ 
    bool operator()(const std::pair<T, U> &lhs, const std::pair<T, U> &rhs) const{ 
     return lhs.first == rhs.first && lhs.second == rhs.second; 
    } 
}; 

struct GraphEdge{ 

}; 


int main(){ 

    std::unordered_map<std::pair<int, int>, 
         std::unique_ptr<GraphEdge>, 
         PairHash<int, int>, 
         PairEqual<int, int>> edges; 

}

Tuttavia, questo mi dà una (ai miei occhi almeno) imperscrutabile errore piuttosto compilatore:

In file included from /usr/include/c++/5/bits/hashtable.h:35:0, 
       from /usr/include/c++/5/unordered_map:47, 
       from prog.cpp:1: 
/usr/include/c++/5/bits/hashtable_policy.h: In instantiation of 'struct std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> >': 
/usr/include/c++/5/type_traits:137:12: required from 'struct std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > >' 
/usr/include/c++/5/type_traits:148:38: required from 'struct std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
/usr/include/c++/5/bits/unordered_map.h:100:66: required from 'class std::unordered_map<std::pair<int, int>, std::unique_ptr<GraphEdge>, PairHash<int, int>, PairEqual<int, int> >' 
prog.cpp:29:43: required from here 
/usr/include/c++/5/bits/hashtable_policy.h:85:34: error: no match for call to '(const PairHash<int, int>) (const std::pair<int, int>&)' 
    noexcept(declval<const _Hash&>()(declval<const _Key&>()))> 
           ^
prog.cpp:7:12: note: candidate: size_t PairHash<T, U>::operator()(const std::pair<_T1, _T2>&) [with T = int; U = int; size_t = unsigned int] <near match> 
    size_t operator()(const std::pair<T, U> &key){ 
      ^
prog.cpp:7:12: note: passing 'const PairHash<int, int>*' as 'this' argument discards qualifiers 
In file included from /usr/include/c++/5/bits/move.h:57:0, 
       from /usr/include/c++/5/bits/stl_pair.h:59, 
       from /usr/include/c++/5/utility:70, 
       from /usr/include/c++/5/unordered_map:38, 
       from prog.cpp:1: 
/usr/include/c++/5/type_traits: In instantiation of 'struct std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >': 
/usr/include/c++/5/bits/unordered_map.h:100:66: required from 'class std::unordered_map<std::pair<int, int>, std::unique_ptr<GraphEdge>, PairHash<int, int>, PairEqual<int, int> >' 
prog.cpp:29:43: required from here 
/usr/include/c++/5/type_traits:148:38: error: 'value' is not a member of 'std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > >' 
    : public integral_constant<bool, !_Pp::value> 
            ^
In file included from /usr/include/c++/5/unordered_map:48:0, 
       from prog.cpp:1: 
/usr/include/c++/5/bits/unordered_map.h: In instantiation of 'class std::unordered_map<std::pair<int, int>, std::unique_ptr<GraphEdge>, PairHash<int, int>, PairEqual<int, int> >': 
prog.cpp:29:43: required from here 
/usr/include/c++/5/bits/unordered_map.h:100:66: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef __umap_hashtable<_Key, _Tp, _Hash, _Pred, _Alloc> _Hashtable; 
                   ^
/usr/include/c++/5/bits/unordered_map.h:107:45: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::key_type key_type; 
              ^
/usr/include/c++/5/bits/unordered_map.h:108:47: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::value_type value_type; 
              ^
/usr/include/c++/5/bits/unordered_map.h:109:48: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::mapped_type mapped_type; 
               ^
/usr/include/c++/5/bits/unordered_map.h:110:43: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::hasher hasher; 
             ^
/usr/include/c++/5/bits/unordered_map.h:111:46: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::key_equal key_equal; 
              ^
/usr/include/c++/5/bits/unordered_map.h:112:51: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::allocator_type allocator_type; 
               ^
/usr/include/c++/5/bits/unordered_map.h:117:45: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::pointer pointer; 
              ^
/usr/include/c++/5/bits/unordered_map.h:118:50: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::const_pointer const_pointer; 
               ^
/usr/include/c++/5/bits/unordered_map.h:119:47: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::reference reference; 
              ^
/usr/include/c++/5/bits/unordered_map.h:120:52: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::const_reference const_reference; 
                ^
/usr/include/c++/5/bits/unordered_map.h:121:46: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::iterator iterator; 
              ^
/usr/include/c++/5/bits/unordered_map.h:122:51: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::const_iterator const_iterator; 
               ^
/usr/include/c++/5/bits/unordered_map.h:123:51: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::local_iterator local_iterator; 
               ^
/usr/include/c++/5/bits/unordered_map.h:124:57: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::const_local_iterator const_local_iterator; 
                 ^
/usr/include/c++/5/bits/unordered_map.h:125:47: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::size_type size_type; 
              ^
/usr/include/c++/5/bits/unordered_map.h:126:52: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     typedef typename _Hashtable::difference_type difference_type; 
                ^
/usr/include/c++/5/bits/unordered_map.h:280:7: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     operator=(initializer_list<value_type> __l) 
    ^
/usr/include/c++/5/bits/unordered_map.h:379:2: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
    emplace(_Args&&... __args) 
^
/usr/include/c++/5/bits/unordered_map.h:432:7: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     insert(const value_type& __x) 
    ^
/usr/include/c++/5/bits/unordered_map.h:439:2: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
    insert(_Pair&& __x) 
^
/usr/include/c++/5/bits/unordered_map.h:499:7: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     insert(initializer_list<value_type> __l) 
    ^
/usr/include/c++/5/bits/unordered_map.h:645:7: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     equal_range(const key_type& __x) 
    ^
/usr/include/c++/5/bits/unordered_map.h:649:7: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<PairHash<int, int> >, std::__detail::__is_noexcept_hash<std::pair<int, int>, PairHash<int, int> > > >' 
     equal_range(const key_type& __x) const 
    ^

Che cosa sto facendo di sbagliato?

fonte

2015-11-08 balajeerc

In una nota a margine, è talmente noioso da decifrare errori del compilatore che coinvolgono stl. Devi leggere un diario per capire quali sono gli errori, il resto sono nomi annidati. Lo stesso problema quando si tenta di leggere un prototipo in Intellisense. Questo è fastidioso. –

Il codice viene compilato per me con VS2015. –

Apparentemente, libstdC++ fa riferimento all'oggetto hash tramite const PairHash<int, int>*. Pertanto, chiamare il numero operator() che non è contrassegnato come const nel programma è un errore del compilatore.

È possibile ottenere il codice da compilare con libstdC++ effettuando operator()const.

A partire da 17.6.3.4 (Requisiti hash), un tipo di hash deve fornire un size_t operator(KeyType) const;, pertanto il codice è effettivamente errato.

fonte

2015-11-08 18:10:22

Non sono sicuro di cosa fare std :: verbiage :: 17.6.3.4 - 2 '[hash.requirements]' ** Chiave data è un tipo di argomento per oggetti funzione di tipo H, in Tabella 26 h è un valore di tipo (possibilmente const) H, u è un lvalue di tipo Key, e k è un valore di un tipo convertibile in (possibilmente const) Key . **. Il "possibilmente const" implica che devi fornire un operatore 'const'()? –

Grazie! È un peccato che non l'abbia capito. Vorrei che l'errore del compilatore fosse un po 'più leggibile. Mi sarei aspettato di essere in grado di correggere un errore del compilatore insignificante. – balajeerc

@CaptainGiraffe Sembra giusto, ho bisogno di leggere più attentamente. Grazie. –

È necessario effettuare il operator() un metodo const nei propri functor personalizzati.

fonte

2015-11-08 18:05:22

Duplica senza aggiungere nulla alla risposta precedente. – rwst

@rwst grazie per il commento. Se controlli le ore penso che troverai che la mia risposta è stata scritta prima –

Creazione di una std :: unordered_map con una chiave std :: pair come chiave

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