C++ hash_function()

The unordered_map::hash_function() method returns a function object the container uses to hash keys. This function determines which bucket an element will be placed in. By default, it uses std::hash<Key>, but it can be customized when the map is declared.

Syntax

unordered_mapName.hash_function();

Parameters:

The hash_function() method takes no parameters.

Return value:

Returns the hash function used by the unordered_map. The return type is the function object used for hashing keys.

Example 1: Default hash function

Get the hash value of a string key using the default hash function:

#include <iostream>
#include <unordered_map>

using namespace std;

int main() {
  unordered_map<string, int> myMap;
  auto hashFunc = myMap.hash_function();

  string key = "gfg";
  cout << "Hash value of key '" << key << "' is: " << hashFunc(key) << endl;
  return 0;
}

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The output of this code is:

Hash value of key 'gfg' is: 2677022477486138405

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This example shows how the internal hash function can be used to compute a hash value for a given key.

Example 2: Comparing hash values of keys

Check how different string keys are hashed:

#include <iostream>
#include <unordered_map>

using namespace std;

int main() {
  unordered_map<string, int> data;
  auto hashFn = data.hash_function();

  cout << "Hash for 'apple': " << hashFn("apple") << endl;
  cout << "Hash for 'banana': " << hashFn("banana") << endl;
  return 0;
}

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The output of the code is:

Hash for 'apple': 7562681486644061055
Hash for 'banana': 680920345727384150

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The hash function generates different values for different strings, helping distribute them across buckets.

Codebyte Example: Hashing integer keys

Visualize how integer keys are hashed in a real-world lookup:

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The hash function often returns the value itself for integer keys, as integers map cleanly to buckets.

Frequently asked questions

1. Can a custom hash function be used in unordered_map?

Yes. A user-defined hash function can be passed as a template parameter when defining the map.

2. Is hash_function() always std::hash?

By default, yes. But if a custom hash is provided during map declaration, hash_function() returns that.

3. When is hash_function() useful?

When debugging or when needing to understand how keys are being distributed across buckets internally.