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# replace_if

### Synopsis

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

### Description

Returns a copy of the original sequence where every type that satisfies the predicate `Pred` has been replaced with `NewType`.

[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/replace_if.hpp>

### Model of

Reversible Algorithm

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`Sequence` | Forward Sequence | An original sequence.
`Pred` | Unary Lambda Expression | A replacement condition.
`NewType` | Any type | A type to replace with.
`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`, an Inserter `in`, and arbitrary type `x`:

    typedef replace_if<s,pred,x,in>::type r;

Return type: | A type.

Semantics:

Equivalent to

    typedef lambda<pred>::type p;
    typedef transform< s, if_< apply_wrap1<p,_1>,x,_1>, in >::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> numbers;
    typedef vector_c<int,1,4,0,2,0,0,3,0> expected;
    typedef replace_if< numbers, greater<_,int_<4> >, int_<0> >::type result;

    BOOST_MPL_ASSERT(( equal< result,expected, equal_to<_,_> > ));

### See also

Transformation Algorithms, Reversible Algorithm, `reverse_replace_if`, `replace`, `remove_if`, `transform`