Oops.. The example in the docs seems to be wrong. It's suppose to be: int main() { foo("bar", 3.14f); foo("baz", value = 6.28f); return 0; } As written in the docs the "name" parameter doesn't get a value, thus the compilation error. To get it to compile, the nullary foo_impl() overload needs to be removed as well. It can never be valid since neither of the arguments have default values. Sorry about that. Rene Rivera wrote:
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 :-(
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#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; }
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-- Daniel Wallin