Question

I am using an unordered_map<float, unsigned short> to implement a hash table in C++.

I know that using floats as keys to a hash table is a bad idea under most circumstances because comparing them is error-prone. However, under these circumstances, I am reading the floats in from large files and their precision is known and constant.

However, I would like to know the details of how unordered_map is hashing my floats in order to estimate collision frequency. I am not overriding the default hash implementation when I create the unordered_map. According to the documentation, the default hash function is std::hash<Key>. Which in my case is std::hash<float>. However, when I look at the std::hash documentation, it is only defined for “template arguments of type char*, const char*, crope, wrope, and the built-in integral types”.

Does anyone know what function is being called to hash the values as I add them to the unordered_map?

unordered_map – http://msdn.microsoft.com/en-us/library/bb982522.aspx

std::hash – http://www.sgi.com/tech/stl/hash.html#1

Answer 1

According to the C++11 standard, float is supported for std::hash as well. The actual hash function is implementation dependent, so even if you can figure out collision frequency for your current compiler a newer version or a different compiler may implement a different hash function. Here is the full list of std::hash specializations:

template <> struct hash<bool>;
template <> struct hash<char>;
template <> struct hash<signed char>;
template <> struct hash<unsigned char>;
template <> struct hash<char16_t>;
template <> struct hash<char32_t>;
template <> struct hash<wchar_t>;
template <> struct hash<short>;
template <> struct hash<unsigned short>;
template <> struct hash<int>;
template <> struct hash<unsigned int>;
template <> struct hash<long>;
template <> struct hash<unsigned long>;
template <> struct hash<long long>;
template <> struct hash<unsigned long long>;
template <> struct hash<float>;
template <> struct hash<double>;
template <> struct hash<long double>;
template <class T> struct hash<T*>;