What's wrong with my hash key type and function?

Question

I'm trying to write my own hash function for a class that is essentially a wrapper for an unsigned integer. I've been trying to follow this thread here: and this resource here. Why doesn't this work? See code comment for errors.

struct Entity
{
    unsigned int id;

    bool operator==( const Entity &other ) const
    {
        return ( id == other.id );
    }
};

template<struct T>
class EntityHash;

template<>
class EntityHash<Entity> // Error: Type name is not allowed
{
public:
    std::size_t operator()( const Entity& entity ) const
    {
        size_t h1 = std::hash<unsigned int>()( entity.id );
        return h1; // Also... do I need a << 1 at the end of this to bitshift even though I'm not combining?
    }
};

// Elsewhere
std::unordered_map<Entity, unsigned int, EntityHash> map; // Error: Argument list for class template EntityHash is missing

About the << 1 thing: If you need it depends on your requirements. The code is correct in both cases (well, that part) — deviantfan
– deviantfan, Commented Oct 21, 2015 at 0:42

aschepler · Accepted Answer · 2015-10-21 00:40:20Z

3

template <struct T>
class EntityHash;

is probably not what you want. Use template <class T> or template <typename T>.

The third template argument of unordered_map must be a type, not the name of a template. So:

std::unordered_map<Entity, unsigned int, EntityHash<Entity>> map;

answered Oct 21, 2015 at 0:40

aschepler

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1 Comment

mdeforge Over a year ago

Thanks, looks like those errors are gone now. However, the compiler is still complaining "The C++ standard doesn't provide a hash for this type." I don't see how that's possible given I'm specifying the third parameter. Does my operator() overload look okay? Just trying to think what else it could be. EDIT: Nevermind, forgot to make the same changes to a different class elsewhere.

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