18

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) ^ /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 ^

What am I doing wrong?

Improve this question
2
  • 2
    On a side note, it is so tedious it is to decipher compiler errors involving stl. You need to read a journal to understand what the errors is, the rest being nested names. Same problem when you try to read a prototype in Intellisense. This is annoying. Commented Nov 8, 2015 at 18:13
  • Your code compiles for me with VS2015. Commented Nov 8, 2015 at 18:17

2 Answers 2

Reset to default
9

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.

Improve this answer
Sign up to request clarification or add additional context in comments.

3 Comments

I'm not sure what to make of the std::verbiage::17.6.3.4 - 2 '[hash.requirements]' Given Key is an argument type for function objects of type H, in Table 26 h is a value of type (possibly const) H, u is an lvalue of type Key, and k is a value of a type convertible to (possibly const) Key.. Does the "possibly const" imply you must supply a const operator()?
Thanks! Its a shame I didn't catch that. I wish the compiler error was a bit more readable. I'd have expected to be able to fix such a trivial compiler error myself.
@CaptainGiraffe Looks right, I need to read more carefully. Thanks.
4

You need to make the operator() a const method in your custom functors.

Improve this answer

2 Comments

Duplicate without anything adding to the previous answer.
@rwst thanks for the comment. If you check the times I think you'll find that my answer was written first

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.