[section:set_union set_union]

[heading Prototype]

``
template<
    class SinglePassRange1,
    class SinglePassRange2,
    class OutputIterator
    >
OutputIterator set_union(const SinglePassRange1& rng1,
                         const SinglePassRange2& rng2,
                         OutputIterator          out);

template<
    class SinglePassRange1,
    class SinglePassRange2,
    class OutputIterator,
    class BinaryPredicate
    >
OutputIterator set_union(const SinglePassRange1& rng1,
                         const SinglePassRange2& rng2,
                         OutputIterator          out,
                         BinaryPredicate         pred);    
 ``

[heading Description]

`set_union` constructs a sorted range that is the union of the sorted ranges `rng1` and `rng2`. The return value is the end of the output range.
The ordering relationship is determined by using `operator<` in the non-predicate versions, and by evaluating `pred` in the predicate versions. 

[heading Definition]

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

[heading Requirements]

[*For the non-predicate versions:]

* `SinglePassRange1` is a model of the __single_pass_range__ Concept.
* `SinglePassRange2` is a model of the __single_pass_range__ Concept.
* `OutputIterator` is a model of the `OutputIteratorConcept`.
* `SinglePassRange1` and `SinglePassRange2` have the same value type.
* `SinglePassRange1`'s value type is a model of the `LessThanComparableConcept`.
* `SinglePassRange2`'s value type is a model of the `LessThanComparableConcept`.
* The ordering of objects of type `SinglePassRange1`'s value type is a [*/strict weak ordering/], as defined in the `LessThanComparableConcept` requirements.
* The ordering of objects of type `SinglePassRange2`'s value type is a [*/strict weak ordering/], as defined in the `LessThanComparableConcept` requirements.

[*For the predicate versions:]

* `SinglePassRange1` is a model of the __single_pass_range__ Concept.
* `SinglePassRange2` is a model of the __single_pass_range__ Concept.
* `OutputIterator` is a model of the `OutputIteratorConcept`.
* `SinglePassRange1` and `SinglePassRange2` have the same value type.
* `BinaryPredicate` is a model of the `StrictWeakOrderingConcept`.
* `SinglePassRange1`'s value type is convertible to `BinaryPredicate`'s first argument type.
* `SinglePassRange2`'s value type is convertible to `BinaryPredicate`'s second argument types.

[heading Precondition:]

[*For the non-predicate versions:]

`rng1` and `rng2` are sorted in ascending order according to `operator<`.

[*For the predicate versions:]

`rng1` and `rng2` are sorted in ascending order according to `pred`.

[heading Complexity]

Linear. `O(N)`, where `N` is `distance(rng1) + distance(rng2)`.

[endsect]