# 27 Algorithms library [algorithms]

## 27.8 Sorting and related operations [alg.sorting]

### 27.8.4 Binary search [alg.binary.search]

#### 27.8.4.4equal_range[equal.range]

```template<class ForwardIterator, class T = iterator_traits<ForwardIterator>::value_type> constexpr pair<ForwardIterator, ForwardIterator> equal_range(ForwardIterator first, ForwardIterator last, const T& value); template<class ForwardIterator, class T = iterator_traits<ForwardIterator>::value_type, class Compare> constexpr pair<ForwardIterator, ForwardIterator> equal_range(ForwardIterator first, ForwardIterator last, const T& value, Compare comp); template<forward_iterator I, sentinel_for<I> S, class Proj = identity, class T = projected_value_t<I, Proj>, indirect_strict_weak_order<const T*, projected<I, Proj>> Comp = ranges::less> constexpr subrange<I> ranges::equal_range(I first, S last, const T& value, Comp comp = {}, Proj proj = {}); template<forward_range R, class Proj = identity, class T = projected_value_t<iterator_t<R>, Proj>, indirect_strict_weak_order<const T*, projected<iterator_t<R>, Proj>> Comp = ranges::less> constexpr borrowed_subrange_t<R> ranges::equal_range(R&& r, const T& value, Comp comp = {}, Proj proj = {}); ```
Let comp be less{} and proj be identity{} for overloads with no parameters by those names.
Preconditions: The elements e of [first, last) are partitioned with respect to the expressions bool(invoke(comp, invoke(proj, e), value)) and !bool(invoke(comp, value, invoke(proj, e))).
Also, for all elements e of [first, last), bool(comp(e, value)) implies !bool(comp(​value, e)) for the overloads in namespace std.
Returns:
• For the overloads in namespace std: {lower_bound(first, last, value, comp), upper_bound(first, last, value, comp)}
• For the overloads in namespace ranges: {ranges::lower_bound(first, last, value, comp, proj), ranges::upper_bound(first, last, value, comp, proj)}
Complexity: At most comparisons and projections.