boost::log::result_of::extract_or_throw — A metafunction that allows to acquire the result of the value extraction.
// In header: <boost/log/attributes/value_extraction_fwd.hpp> template<typename T, typename TagT> struct extract_or_throw { };
The metafunction results in a type that is in form of T const&, if T is not an MPL type sequence, or value_ref< T, TagT > otherwise. In the latter case the value reference shall never be empty.
T const&
T
value_ref< T, TagT >