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# fold

### Synopsis

    template<
          typename Sequence
        , typename State
        , typename ForwardOp
        >
    struct fold
    {
        typedef unspecified type;
    };

### Description

Returns the result of the successive application of binary `ForwardOp` to the result of the previous `ForwardOp` invocation (`State` if it's the first call) and every element of the sequence in the range [`begin<Sequence>::type`, `end<Sequence>::type`) in order.

### Header

    #include <boost/mpl/fold.hpp>

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`Sequence` | Forward Sequence | A sequence to iterate.
`State` | Any type | The initial state for the first `ForwardOp` application.
`ForwardOp` | Binary Lambda Expression | The operation to be executed on forward traversal.

### Expression semantics

For any Forward Sequence `s`, binary Lambda Expression `op`, and arbitrary type `state`:

    typedef fold<s,state,op>::type t;

Return type: | A type.
Semantics: | Equivalent to
    typedef iter_fold< s,state,apply<op,_1,deref<_2> > >::type t;

### Complexity

Linear. Exactly `size<s>::value` applications of `op`.

### Example

    typedef vector<long,float,short,double,float,long,long double> types;
    typedef fold<
          types
        , int_<0>
        , if_< is_float<_2>,next<_1>,_1 >
        >::type number_of_floats;

    BOOST_MPL_ASSERT_RELATION( number_of_floats::value, ==, 4 );

### See also

Algorithms, `accumulate`, `reverse_fold`, `iter_fold`, `reverse_iter_fold`, `copy`, `copy_if`