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## Function dynamic_any_cast

boost::type_erasure::dynamic_any_cast

## Synopsis

    // In header: <boost/type_erasure/dynamic_any_cast.hpp>

    template<typename R, typename Any> R dynamic_any_cast(Any && arg);
    template<typename R, typename Any, typename Map>
      R dynamic_any_cast(Any && arg, const static_binding< Map > &);

## Description

Downcasts or crosscasts an `any`.

The single argument form can only be used when `R` uses a single non-deduced placeholder.

Example:

    // Assume that typeid_<>, copy_constructible<>, and incrementable<>
    // have all been registered for int.
    any<mpl::vector<typeid_<>, copy_constructible<> > > x(1);
    typedef any<
        mpl::vector<
            typeid_<>,
            copy_constructible<>,
            incrementable<>
        >
    > incrementable_any;
    auto y = dynamic_any_cast<incrementable_any>(x);
    ++y;
    assert(any_cast<int>(y) == 2);

Requires: | `R` and `Any` must both be specializations of any. PlaceholderMap must be an MPL map with a key for every non-deduced placeholder used by R. The value associated with each key should be the corresponding placeholder in Any. The concept of Any must include typeid_, for every placeholder which is used by R.
Throws: | bad_any_cast if the concepts used by R were not previously registered via a call to register_binding.