equal_range

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##### equal_range

###### Prototype

    template<
        class ForwardRange,
        class Value
        >
    std::pair<typename range_iterator<ForwardRange>::type,
              typename range_iterator<ForwardRange>::type>
    equal_range(ForwardRange& rng, const Value& val);

    template<
        class ForwardRange,
        class Value
        >
    std::pair<typename range_iterator<const ForwardRange>::type,
              typename range_iterator<const ForwardRange>::type>
    equal_range(const ForwardRange& rng, const Value& val);

    template<
        class ForwardRange,
        class Value,
        class SortPredicate
        >
    std::pair<typename range_iterator<ForwardRange>::type,
              typename range_iterator<ForwardRange>::type>
    equal_range(ForwardRange& rng, const Value& val, SortPredicate pred);

    template<
        class ForwardRange,
        class Value,
        class SortPredicate
        >
    std::pair<typename range_iterator<const ForwardRange>::type,
              typename range_iterator<const ForwardRange>::type>
    equal_range(const ForwardRange& rng, const Value& val, SortPredicate pred);

###### Description

`equal_range` returns a range in the form of a pair of iterators where all of the elements are equal to `val`. If no values are found that are equal to `val`, then an empty range is returned, hence `result.first == result.second`. For the non-predicate versions of `equal_range` the equality of elements is determined by `operator<`. For the predicate versions of `equal_range` the equality of elements is determined by `pred`.

###### Definition

Defined in the header file `boost/range/algorithm/equal_range.hpp`

###### Requirements

For the non-predicate versions:

- `ForwardRange` is a model of the Forward Range Concept.
- `Value` is a model of the `LessThanComparableConcept`.
- The ordering of objects of type `Value` is a strict weak ordering, as defined in the `LessThanComparableConcept` requirements.
- `ForwardRange`'s value type is the same type as `Value`.

For the predicate versions:

- `ForwardRange` is a model of the Forward Range Concept.
- `SortPredicate` is a model of the `StrictWeakOrderingConcept`.
- `ForwardRange`'s value type is the same as `Value`.
- `ForwardRange`'s value type is convertible to both of `SortPredicate`'s argument types.

###### Precondition:

For the non-predicate versions: `rng` is ordered in ascending order according to `operator<`.

For the predicate versions: `rng` is ordered in ascending order according to `pred`.

###### Complexity

For random-access ranges, the complexity is `O(log N)`, otherwise the complexity is `O(N)`.