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# copy_if

### Synopsis

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

### Description

Returns a filtered copy of the original sequence containing the elements 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/copy_if.hpp>

### Model of

Reversible Algorithm

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`Sequence` | Forward Sequence | A sequence to copy.
`Pred` | Unary Lambda Expression | A copying 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`, an unary Lambda Expression `pred`, and an Inserter `in`:

    typedef copy_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>
            , apply_wrap2<op,_1,_2>
            , identity<_1>
            >
        >::type r;

### Complexity

Linear. Exactly `size<s>::value` applications of `pred`, and at most `size<s>::value` applications of `in::operation`.

### Example

    typedef copy_if<
          range_c<int,0,10>
        , less< _1, int_<5> >
        , back_inserter< vector<> >
        >::type result;

    BOOST_MPL_ASSERT_RELATION( size<result>::value, ==, 5 );
    BOOST_MPL_ASSERT(( equal<result,range_c<int,0,5> > ));

### See also

Transformation Algorithms, Reversible Algorithm, reverse_copy_if, copy, remove_if, replace_if