any_range

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#### any_range

###### Description

`any_range` is a range that has the type information erased hence a `any_range<int, boost::forward_traversal_tag, int, std::ptrdiff_t>` can be used to represent a `std::vector<int>`, a `std::list<int>` or many other types.

The type erasure article covers the motivation and goals of type erasure in this context. Clearly my implementation is building upon a lot of prior art created by others. Thomas Becker's `any_iterator` was a strong influence. Adobe also have an `any_iterator` implementation, but this has very tight coupling to other parts of the library that precluded it from use in Boost.Range. Early development versions of this Range Adaptor directly used Thomas Becker's any_iterator implementation. Subsequently I discovered that the heap allocations of this and many other implementations cause poor speed performance particularly at the tails of the distribution. To solve this required a new design that incorporated the embedded buffer optimization.

Despite the underlying `any_iterator` being the fastest available implementation, the performance overhead of `any_range` is still appreciable due to the cost of virtual function calls required to implement `increment`, `decrement`, `advance`, `equal` etc. Frequently a better design choice is to convert to a canonical form.

Please see the type_erased for a Range Adaptor that returns `any_range` instances.

###### Synopsis

    template<
        class Value
      , class Traversal
      , class Reference
      , class Difference
      , class Buffer = any_iterator_default_buffer
    >
    class any_range
        : public iterator_range<
            range_detail::any_iterator<
                Value
              , Traversal
              , Reference
              , Difference
              , Buffer
            >
        >
    {
        typedef range_detail::any_iterator<
            Value
          , Traversal
          , Reference
          , Difference
          , Buffer
        > any_iterator_type;

        typedef iterator_range<any_iterator_type> base_type;

        struct enabler {};
        struct disabler {};
    public:
        typedef any_iterator_type iterator;
        typedef any_iterator_type const_iterator;

        any_range()
        {
        }

        any_range(const any_range& other)
            : base_type(other)
        {
        }

        template<class WrappedRange>
        any_range(WrappedRange& wrapped_range)
        : base_type(boost::begin(wrapped_range),
                    boost::end(wrapped_range))
        {
        }

        template<class WrappedRange>
        any_range(const WrappedRange& wrapped_range)
        : base_type(boost::begin(wrapped_range),
                    boost::end(wrapped_range))
        {
        }

        template<
            class OtherValue
          , class OtherTraversal
          , class OtherReference
          , class OtherDifference
        >
        any_range(const any_range<
                            OtherValue
                          , OtherTraversal
                          , OtherReference
                          , OtherDifference
                          , Buffer
                        >& other)
        : base_type(boost::begin(other), boost::end(other))
        {
        }

        template<class Iterator>
        any_range(Iterator first, Iterator last)
            : base_type(first, last)
        {
        }
    };

###### Definition

Defined in header file `boost/range/any_range.hpp`