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Struct template extract_or_default

boost::log::result_of::extract_or_default — 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 DefaultT, typename TagT> 
struct extract_or_default {


The metafunction results in a type that is in form of T const&, if T is not an MPL type sequence and DefaultT is the same as T, or value_ref< TypesT, TagT > otherwise, with TypesT being a type sequence comprising the types from sequence T and DefaultT, if it is not present in T already.