What kind of restrictions are there on the datatype of key and value when constructing an unordered_map in C++11?
I tried to create this:
unordered_map<vector<int>, int>
This gave me a compilation error. Do I need to write my own hasher?
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@MarcoA. I also want to find out "when" to write a custom hasher. – ibp73 Oct 03 '14 at 09:25
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1When you have a type for which it isn't defined or unless you're not satisfied (for whatever reason) with the default one – Marco A. Oct 03 '14 at 09:27
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@MarcoA. How do I find out the types for which it isn't defined? I couldn't find a comprehensive list of basic types anywhere. – ibp73 Oct 03 '14 at 10:37
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2This is a _good_ question, and it is not the same as asking "how do I define a custom hash function for `unordered_map`?" – Lightness Races in Orbit Oct 03 '14 at 11:00
1 Answers
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std::unordered_map's keys need a hash implementation by specialising std::hash.
The standard specializations for basic types which are defined in the STL are:
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<long long>;
template<> struct hash<unsigned 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*>;
For everything else you need to write your own hash and/or use boost::hash.
Furthermore as Tony D's comment says:
You can specify the hash function as a third template parameter if you prefer. Separately, operator== must also be available for the key objects, or comparison specified as a fourth template parameter.
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3+1 for some useful information, but "by specialising `std::hash`" isn't quite true, as you can specify the hash function as a third template parameter if you prefer. Separately, `==` must also be available for the key objects, or comparison specified as a fourth template parameter.... – Tony Delroy Oct 03 '14 at 10:57