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# unique

### Synopsis

    template<
          typename Seq
        , typename Pred
        , typename In = unspecified
        >
    struct unique
    {
        typedef unspecified type;
    };

### Description

Returns a sequence of the initial elements of every subrange of the original sequence `Seq` whose elements are all the same.

[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/unique.hpp>

### Model of

Reversible Algorithm

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`Sequence` | Forward Sequence | An original sequence.
`Pred` | Binary Lambda Expression | An equivalence relation.
`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`, a binary Lambda Expression `pred`, and an Inserter `in`:

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

Return type: | A type.

Semantics:

If `size<s>::value <= 1`, then equivalent to

    typedef copy<s,in>::type r;

otherwise equivalent to

    typedef lambda<pred>::type p;
    typedef lambda<in::operation>::type in_op;
    typedef apply_wrap2<
          in_op
        , in::state
        , front<types>::type
        >::type in_state;

    typedef fold<
          s
        , pair< in_state, front<s>::type >
        , eval_if<
              apply_wrap2<p, second<_1>, _2>
            , identity< first<_1> >
            , apply_wrap2<in_op, first<_1>, _2>
            >
        >::type::first r;

### Complexity

Linear. Performs exactly `size<s>::value - 1` applications of `pred`, and at most `size<s>::value` insertions.

### Example

    typedef vector<int,float,float,char,int,int,int,double> types;
    typedef vector<int,float,char,int,double> expected;
    typedef unique< types, is_same<_1,_2> >::type result;

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

### See also

Transformation Algorithms, Reversible Algorithm, `reverse_unique`, `remove`, `copy_if`, `replace_if`