Unordered Associative Containers (C++) - Custom Hash Functions

Custom Hash Functions

In order to use custom objects in std::unordered_map, a custom hash function must be defined. This function takes a const reference to the custom type and returns a size_t

struct X{int i,j,k;}; struct hash_X{ size_t operator(const X &x) const{ return hash(x.i) ^ hash(x.j) ^ hash(x.k); } };

The user defined function can be used as is in std::unordered_map, by passing it as a template parameter

std::unordered_map my_map;

Or can be set as the default hash function by specializing the std::hash function

namespace std { template <> class hash{ public : size_t operator(const X &x ) const{ return hash(x.i) ^ hash(x.j) ^ hash(x.k); } }; } //... std::unordered_map my_map;

Read more about this topic:  Unordered Associative Containers (C++)

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