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##### filter_if

###### Description

Returns the result type of `filter_if` given the sequence and unary MPL Lambda Expression predicate type.

###### Synopsis

    template<
        typename Sequence,
        typename Pred
        >
    struct filter_if
    {
        typedef unspecified type;
    };

Table 1.84. Parameter

Parameter | Requirement | Description
--- | --- | ---
`Sequence` | A model of Forward Sequence | Operation's argument
`Pred` | A unary MPL Lambda Expression | Type to retain

###### Expression Semantics

    result_of::filter_if<Sequence, Pred>::type

Return type:

- A model of Forward Sequence.
- A model of Associative Sequence if `Sequence` implements the Associative Sequence model.

Semantics: Returns a sequence containing the elements of `Sequence` for which `Pred` evaluates to `boost::mpl::true_`.

###### Complexity

Constant.

###### Header

    #include <boost/fusion/algorithm/transformation/filter_if.hpp>
    #include <boost/fusion/include/filter_if.hpp>