template<typename Value, bool IsIndexable = is_indexable<Value>::value>
struct boost::geometry::index::detail::indexable< Value, IsIndexable >
The function object extracting Indexable from Value. 
It translates Value object to Indexable object. The default version handles Values which are Indexables. This template is also specialized for std::pair<Indexable, T2>, boost::tuple<Indexable, ...> and std::tuple<Indexable, ...>.
- Template Parameters
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    | Value | The Value type which may be translated directly to the Indexable. |  | IsIndexable | If true, the const reference to Value is returned. |