[Entry] C++ unordered map: hash_function()

content/cpp/concepts/unordered-map/terms/hash-function/hash-function.md

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+---
+Title: 'hash_function()'
+Description: 'Returns the hash function used internally by an unordered_map to map keys to buckets.'
+Subjects:
+  - 'Computer Science'
+  - 'Data Science'
+Tags:
+  - 'Hash Maps'
+  - 'STL'
+CatalogContent:
+  - 'learn-cpp'
+  - 'paths/computer-science'
+---
+
+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
+
+```pseudo
+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:
+
+```cpp
+#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;
+}
+```
+
+The output of this code is:
+
+```shell
+Hash value of key 'gfg' is: 2677022477486138405
+```
+
+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:
+
+```cpp
+#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;
+}
+```
+
+The output of the code is:
+
+```shell
+Hash for 'apple': 7562681486644061055
+Hash for 'banana': 680920345727384150
+```
+
+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:
+
+```codebyte/cpp
+#include <iostream>
+#include <unordered_map>
+
+using namespace std;
+
+int main() {
+  unordered_map<int, string> lookup;
+  lookup[101] = "Alice";
+  lookup[202] = "Bob";
+
+  auto h = lookup.hash_function();
+  cout << "Hash of 101: " << h(101) << "\n";
+  cout << "Hash of 202: " << h(202) << "\n";
+  return 0;
+}
+```
+
+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.