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> #include <boost/type_traits/is_base_of.hpp>
> #include <boost/utility/enable_if.hpp>
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> 	
> 	// Implicit conversion from child-type reference_wrapper
> 	template <typename FromT>
> 	reference_wrapper(reference_wrapper<FromT> const& wrapper,typename boost::enable_if<boost::is_base_of<T,FromT> >::type* =0): t_(wrapper.t_) {}
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> 	
> 	// Implicit conversion to parent-type references
> 	template <typename ToT>
> 	operator typename boost::enable_if<boost::is_base_of<ToT,T>,ToT>::type () const { return *t_ ; }
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> 	template <typename OtherT> friend class reference_wrapper;