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Showing content from https://cplusplus.com/reference/unordered_map/unordered_multimap/hash_function/ below:

public member type

<unordered_map>

hasher hash_function() const;

Get hash function

The hash function is a unary function that takes an object of type key_type as argument and returns a unique value of type size_t based on it. It is adopted by the container on construction (see unordered_multimap's constructor for more info). By default, it is the default hashing function for the corresponding key type: hash<key_type>.

Member type hasher is the type of the hash function used by the container, defined in unordered_multimap as an alias of its third template parameter (Hash).

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// unordered_multimap::hash_function
#include <iostream>
#include <string>
#include <unordered_map>

typedef std::unordered_multimap<std::string,std::string> stringmap;

int main ()
{
  stringmap myumm;

  stringmap::hasher fn = myumm.hash_function();

  std::cout << "this: " << fn ("this") << std::endl;
  std::cout << "thin: " << fn ("thin") << std::endl;

  return 0;
}

this: 671344778
thin: 3223852919

Notice how two similar strings yield quite different hash values.

unordered_multimap::key_eq

Get key equivalence predicate (public member type)

unordered_multimap::get_allocator

Get allocator (public member function)

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