# operator==,!=(std::unordered_set)

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< cpp‎ | container‎ | unordered set

template< class Key, class Hash, class KeyEqual, class Alloc > bool operator==( const std::unordered_set<Key, Hash, KeyEqual, Alloc>& lhs, const std::unordered_set<Key, Hash, KeyEqual, Alloc>& rhs ); | (1) |
template< class Key, class Hash, class KeyEqual, class Alloc > bool operator!=( const std::unordered_set<Key, Hash, KeyEqual, Alloc>& lhs, const std::unordered_set<Key, Hash, KeyEqual, Alloc>& rhs ); | (2) | (until C++20)

Compares the contents of two unordered containers.

The contents of two unordered containers `lhs` and `rhs` are equal if the following conditions hold:

- lhs.size() == rhs.size()
- each group of equivalent elements `[lhs_eq1, lhs_eq2)` obtained from lhs.equal_range(lhs_eq1) has a corresponding group of equivalent elements in the other container `[rhs_eq1, rhs_eq2)` obtained from rhs.equal_range(rhs_eq1), that has the following properties:

  - std::distance(lhs_eq1, lhs_eq2) == std::distance(rhs_eq1, rhs_eq2).
  - std::is_permutation(lhs_eq1, lhs_eq2, rhs_eq1) == true.

The behavior is undefined if `Key` is not EqualityComparable.

The behavior is also undefined if `hash_function()` and `key_eq()` do (until C++20)`key_eq()` does (since C++20) not have the same behavior on `lhs` and `rhs` or if operator== for `Key` is not a refinement of the partition into equivalent-key groups introduced by `key_eq()` (that is, if two elements that compare equal using operator== fall into different partitions)

The `!=` operator is synthesized from `operator==`. | (since C++20)

### Parameters

lhs, rhs | - | unordered containers to compare

### Return value

1) true if the contents of the containers are equal, false otherwise
2) true if the contents of the containers are not equal, false otherwise

### Complexity

Proportional to N calls to operator== on `value_type`, calls to the predicate returned by `key_eq`, and calls to the hasher returned by `hash_function`, in the average case, proportional to N2 in the worst case where N is the size of the container.

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