如何为模板类实现std :: hash

Question

I have a template class looking like this:

template <int N, class TypeId> class Indexer {
...
}

and I want to use it in std::unordered_map , I need a hash function. In the codebase we already had something similar (but on an untemplated class) so I tried to do it like this:

namespace std {
template <int N, class TypeId>
struct hash<Indexer<N, TypeId> > {
    size_t operator()(const Indexer<N, TypeId>& id) const noexcept {
        ...
    }
};
}

It is also quite similar to another answer . Unfortunately this does not work, just gives a bunch of unhelpfull errors. Any insights?

Answer 1

It looks like you're missing a semi-colon at the end of the definition of the Indexer class.

This works:

#include <functional>

template <int N, class TypeId> struct Indexer {};

namespace std {
template <int N, class TypeId>
struct hash<Indexer<N, TypeId> > {
   size_t operator()(const Indexer<N, TypeId>& id) const noexcept { return 0; }
};
}

int main() {
   return 0;
}