Term Entry - C++ Unordered-sets - key_eq()

content/cpp/concepts/unordered-set/terms/key-eq/key-eq.md

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+---
+Title: 'key_eq()'
+Description: 'Returns the key equality comparison function used by the unordered set.'
+Subjects:
+  - 'Code Foundations'
+  - 'Computer Science'
+Tags:
+  - 'Comparison'
+  - 'Containers'
+  - 'Equality'
+  - 'Functions'
+  - 'Hashes'
+  - 'Sets'
+  - 'STL'
+CatalogContent:
+  - 'learn-c-plus-plus'
+  - 'paths/computer-science'
+---
+
+The **`key_eq()`** method returns the equality comparison function object used by an [`unordered_set`](https://www.codecademy.com/resources/docs/cpp/unordered-set) to determine whether two keys are considered equal.
+
+## Syntax
+
+```pseudo
+unordered_set_name.key_eq();
+```
+
+**Parameters:**
+
+This method takes no parameters.
+
+**Return value:**
+
+Returns a `key_equal` function object. By default, this is `std::equal_to<T>`, which compares keys using the `==` operator.
+
+## Example
+
+The following example illustrates retrieving and using the equality comparison function from an `unordered_set`:
+
+```cpp
+#include<iostream>
+#include<unordered_set>
+
+using namespace std;
+
+int main() {
+  unordered_set<int> numbers = {1, 2, 3};
+
+  auto eq = numbers.key_eq();
+
+  cout << eq(2, 2) << "\n";
+  cout << eq(2, 3) << "\n";
+
+  return 0;
+}
+```
+
+The above program gives the following output:
+
+```shell
+1
+0
+```
+
+> **Note:** If two keys are considered equal by `key_eq()`, they must also produce the same hash value. Failing to maintain this consistency results in undefined behavior.
+
+## Codebyte Example
+
+In this example, a custom case-insensitive equality function is used with `unordered_set`, and `key_eq()` retrieves that function to compare two strings while ensuring duplicate keys are not inserted:
+
+```codebyte/cpp
+#include<iostream>
+#include<unordered_set>
+#include<string>
+#include<cctype>
+
+using namespace std;
+
+struct CaseInsensitiveHash {
+  size_t operator()(const string& str) const {
+  size_t hash = 0;
+
+  for(char ch : str) {
+    hash = hash * 31 + tolower(ch);
+  }
+
+  return hash;
+  }
+};
+
+struct CaseInsensitiveEq {
+  bool operator()(const string& a, const string& b) const {
+    if(a.size() != b.size())
+      return false;
+
+    for(size_t i = 0; i < a.size(); i++) {
+      if(tolower(a[i]) != tolower(b[i]))
+        return false;
+    }
+
+    return true;
+  }
+};
+
+int main() {
+  unordered_set<string, CaseInsensitiveHash, CaseInsensitiveEq> words;
+
+  words.insert("Codecademy");
+  words.insert("codecademy"); // will be considered equal, and not inserted
+
+  auto eq = words.key_eq();
+
+  cout << eq("Codecademy", "codecademy") << "\n";
+  cout << words.size() << "\n";
+
+  return 0;
+}
+```