I tried to follow along with the docs... But I can't get very far. With CW8.3 I get: ### mwcc Compiler: # File: named_params_ex1.cpp # ----------------------------- # 42: std::cout << params[name] << " = " << params[value] << "\n"; # Error: ^ # pointer/array required # (point of instantiation: 'foo()') # (instantiating: 'foo_impl<boost::detail::nil>(const boost::detail::nil &)') And to see if it was just a CW problem, I also tried VC71: ..\..\temp\named_params\named_params_ex1.cpp(42) : error C2784: 'Default::result_type boost::detail::nil::operator [](const boost::detail::lazy_named_default<K,Default> &) const' : could not deduce template argument for 'const boost::detail::lazy_named_default<KW,DefaultFn> &' from 'boost::keyword<Tag>' with [ Tag=name_t ] C:\CVSROOTs\Boost-Sandbox\boost-sandbox\boost\named_params.hpp(186) : see declaration of 'boost::detail::nil::operator`[]'' ..\..\temp\named_params\named_params_ex1.cpp(24) : see reference to function template instantiation 'void foo_impl<boost::detail::nil>(const Params &)' being compiled with [ Params=boost::detail::nil ] ..\..\temp\named_params\named_params_ex1.cpp(42) : error C2784: 'Default &boost::detail::nil::operator [](const boost::detail::named_default<K,Default> &) const' : could not deduce template argument for 'const boost::detail::named_default<KW,Default> &' from 'boost::keyword<Tag>' with [ Tag=name_t ] C:\CVSROOTs\Boost-Sandbox\boost-sandbox\boost\named_params.hpp(172) : see declaration of 'boost::detail::nil::operator`[]'' ..\..\temp\named_params\named_params_ex1.cpp(42) : error C2676: binary '[' : 'const boost::detail::nil' does not define this operator or a conversion to a type acceptable to the predefined operator ..\..\temp\named_params\named_params_ex1.cpp(42) : error C2784: 'Default::result_type boost::detail::nil::operator [](const boost::detail::lazy_named_default<K,Default> &) const' : could not deduce template argument for 'const boost::detail::lazy_named_default<KW,DefaultFn> &' from 'boost::keyword<Tag>' with [ Tag=value_t ] C:\CVSROOTs\Boost-Sandbox\boost-sandbox\boost\named_params.hpp(186) : see declaration of 'boost::detail::nil::operator`[]'' ..\..\temp\named_params\named_params_ex1.cpp(42) : error C2784: 'Default &boost::detail::nil::operator [](const boost::detail::named_default<K,Default> &) const' : could not deduce template argument for 'const boost::detail::named_default<KW,Default> &' from 'boost::keyword<Tag>' with [ Tag=value_t ] C:\CVSROOTs\Boost-Sandbox\boost-sandbox\boost\named_params.hpp(172) : see declaration of 'boost::detail::nil::operator`[]'' ..\..\temp\named_params\named_params_ex1.cpp(42) : error C2676: binary '[' : 'const boost::detail::nil' does not define this operator or a conversion to a type acceptable to the predefined operator Makes it's kinda hard to understand when it fails like this :-( #include <boost/named_params.hpp> #include <iostream> struct name_t; // tag types struct value_t; namespace { boost::keyword<name_t> name; // keyword objects boost::keyword<value_t> value; } struct foo_keywords : boost::keywords< name_t , value_t > {}; template<class Params> void foo_impl(const Params&); void foo() { foo_impl(foo_keywords()()); } template<class A0> void foo(const A0& a0) { foo_impl(foo_keywords()(a0)); } template<class A0, class A1> void foo(const A0& a0, const A1& a1) { foo_impl(foo_keywords()(a0, a1)); } template<class Params> void foo_impl(const Params& params) { std::cout << params[name] << " = " << params[value] << "\n"; } int main() { foo("bar", 3.14f); foo(value = 6.28f, "baz"); return 0; }