The following patch to boost/lambda/detail/return_type_traits.hpp enables it to extract the return type information from a nested ::result_type typedef instead of insisting on a sig<> template. This is useful since all standard function objects in <functional> and many user-defined function objects (including those produced by boost::mem_fn) have result_type but no sig<>. I have not commited the patch yet, even though it seems to work fine for me, since it alters the Lambda behavior in the case where a function object defines both result_type and sig<>. Currently, sig<> is used. With the patch, result_type would override sig<>. All function objects known to man that have both do define sig<> to return the same type as result_type; nevertheless, I'd like confirmation from the maintainers first, before I commit. Thanks for listening; patch follows: *** 17,22 **** --- 17,24 ---- #ifndef BOOST_LAMBDA_RETURN_TYPE_TRAITS_HPP #define BOOST_LAMBDA_RETURN_TYPE_TRAITS_HPP + #include "boost/mpl/aux_/has_xxx.hpp" + #include <cstddef> // needed for the ptrdiff_t namespace boost { *************** *** 239,244 **** --- 241,265 ---- typedef Ret type; }; + // ::result_type support + + namespace detail + { + + BOOST_MPL_HAS_XXX_TRAIT_DEF(result_type) + + template<class F> struct get_result_type + { + typedef typename F::result_type type; + }; + + template<class F, class A> struct get_sig + { + typedef typename function_adaptor<F>::template sig<A>::type type; + }; + + } // namespace detail + // Ret is detail::unspecified, so try to deduce return type template<int I, class Args> struct return_type_N<function_action<I, detail::unspecified>, Args > { *************** *** 251,257 **** public: // pass the function to function_adaptor, and get the return type from // that ! typedef typename function_adaptor<plain_Func>::template sig<Args>::type type; }; --- 272,282 ---- public: // pass the function to function_adaptor, and get the return type from // that ! typedef typename detail::IF< ! detail::has_result_type<plain_Func>::value, ! detail::get_result_type<plain_Func>, ! detail::get_sig<plain_Func, Args> ! >::RET::type type; }; -- Peter Dimov http://www.pdimov.com