c++ - Creating an std::unordered_map with an std::pair as key

Question

I am trying to create an std::unordered_map with an std::pair as key. As you can imagine, this would require me to explicitly provide a class to generate a hash for a given key, as well as an equality comparator for the keys. Here's my code so far:

#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; 

}

However, this gives me a rather (to my eyes at least) inscrutable compiler error:

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)

Answer 1

Apparently, libstdc++ refers to your hash object via const PairHash<int, int>*. Thus calling the operator () which is not marked const in your program is a compiler error.

You can get your code to compile with libstdc++ by making operator() const.

As of 17.6.3.4 (Hash Requirements), a Hash type must provide a size_t operator(KeyType) const;, so your code is indeed incorrect.