This seems to work: namespace boost { namespace accumulators { namespace impl { template<typename Sample> struct abs_sqr_accumulator : accumulator_base { typedef typename Sample::value_type result_type; template<typename Args> abs_sqr_accumulator (Args const& args) {} template<typename Args> void operator () (Args const& args) { this->sum += real (args[sample] * conj (args[sample])); } template<typename Args> result_type result (Args const& args) const { return boost::numeric::average (this->sum, count (args)); } result_type sum; }; }}} namespace boost { namespace accumulators { namespace tag { struct abs_sqr : depends_on< count > // depends_on<> to specify dependencies { // Define a nested typedef called 'impl' that specifies which // accumulator implements this feature. typedef accumulators::impl::abs_sqr_accumulator< mpl::_1 > impl; }; }}} namespace boost { namespace accumulators { namespace extract { extractor< tag::abs_sqr> const abs_sqr = {}; } using extract::abs_sqr; } }