CODEWITHSUNDEEP
c++hashunordered-mapstd-pair
Aleksey Kontsevich
Aleksey Kontsevich

Reputation: 4991

Build fails for unordered_map with pair as a key (C++)

I see it is possible ot use std::pair as a key for std::unordered_map. In my case I need to use std::type_index in the pair. However have some problems building it. My code is:

template<class Base, class Result, bool Commutative>
struct Multimethod2
{
    using Args = std::pair<std::type_index, std::type_index>;
    using Method = std::function<bool(Base *, Base *)>;

    struct ArgsHash {
        std::size_t operator () (Args &p) const {
            std::size_t h1 = std::hash<std::type_index>()(p.first);
            std::size_t h2 = std::hash<std::type_index>()(p.second);
            return h1 ^ h2;
        }
    };

    struct KeyEqual
    {
        bool operator()(const Args &a1, const Args &a2) const
        {
            return (a1.first == a2.first && a1.second == a2.second) ||
                   (a1.first == a2.second && a1.second == a2.first);
        }
    };

    std::unordered_map<Args, Method, ArgsHash, KeyEqual> methods;
...
}

Got errors:

/usr/include/c++/7/bits/hashtable_policy.h:87: error: no match for call to ‘(const Multimethod2<Shape, bool, true>::ArgsHash) (const std::pair<std::type_index, std::type_index>&)’
  noexcept(declval<const _Hash&>()(declval<const _Key&>()))>
          ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~

/usr/include/c++/7/bits/hashtable_policy.h:87: error: binding reference of type ‘Multimethod2<Shape, bool, true>::Args& {aka std::pair<std::type_index, std::type_index>&}’ to ‘const std::pair<std::type_index, std::type_index>’ discards qualifiers
  noexcept(declval<const _Hash&>()(declval<const _Key&>()))>
          ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~

/usr/include/c++/7/type_traits:154: error: ‘value’ is not a member of ‘std::__and_<std::__is_fast_hash<Multimethod2<Shape, bool, true>::ArgsHash>, std::__detail::__is_noexcept_hash<std::pair<std::type_index, std::type_index>, Multimethod2<Shape, bool, true>::ArgsHash> >’
    : public integral_constant<bool, !_Pp::value>
                                      ^~~~

...

What is wrong with syntax here?

Upvotes: 3

Views: 1369

Answers (1)

Massimiliano Janes
Massimiliano Janes

Reputation: 5624

As per hash requirements, ArgsHash::operator() should take Args by const&.

By the way, your hash function is probably bad ( what happens when you have two identical type_index ? )

Combining hashes is not trivial (there's a reason why there's no std::hash_combine); anyway, you may want to give a try to boost.hash_combine for a ready made more-or-less general purpose solution...

Upvotes: 4

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