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Front Page / Algorithms / Transformation Algorithms / copy_if

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