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# remove_if

### Synopsis

    template<
          typename Sequence
        , typename Pred
        , typename In = unspecified
        >
    struct remove_if
    {
        typedef unspecified type;
    };

### Description

Returns a new sequence that contains all the elements from [`begin<Sequence>::type`, `end<Sequence>::type`) range except those that satisfy the predicate `Pred`.

[Note: This wording applies to a no-inserter version(s) of the algorithm. See the Expression semantics subsection for a precise specification of the algorithm's details in all cases — end note]

### Header

    #include <boost/mpl/remove_if.hpp>

### Model of

Reversible Algorithm

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`Sequence` | Forward Sequence | An original sequence.
`Pred` | Unary Lambda Expression | A removal condition.
`In` | Inserter | An inserter.

### Expression semantics

The semantics of an expression are defined only where they differ from, or are not defined in Reversible Algorithm.

For any Forward Sequence `s`, and an Inserter `in`, and an unary Lambda Expression `pred`:

    typedef remove_if<s,pred,in>::type r;

Return type: | A type.
Semantics: | Equivalent to
    typedef lambda<pred>::type p;
    typedef lambda<in::operation>::type op;

    typedef fold<
          s
        , in::state
        , eval_if<
              apply_wrap1<p,_2>
            , identity<_1>
            , apply_wrap2<op,_1,_2>
            >
        >::type r;

### Complexity

Linear. Performs exactly `size<s>::value` applications of `pred`, and at most `size<s>::value` insertions.

### Example

    typedef vector_c<int,1,4,5,2,7,5,3,5>::type numbers;
    typedef remove_if< numbers, greater<_,int_<4> > >::type result;

    BOOST_MPL_ASSERT(( equal< result,vector_c<int,1,4,2,3>,equal_to<_,_> > ));

### See also

Transformation Algorithms, Reversible Algorithm, reverse_remove_if, remove, copy_if, replace_if