[boost] Re: [Review] Named parameters library - example doesn't work

From: Daniel Wallin (dalwan01_at_[hidden])
Date: 2004-11-14 16:45:35

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 :-(
> 
> 
> 
> ------------------------------------------------------------------------
> 
> #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