Re: [Boost-users] [parameter] A few usage questions

From: John Maddock (john_at_[hidden])
Date: 2007-05-27 07:51:25

John Maddock wrote:
>> From within the body of the BOOST_PARAMETER_FUNCTION generated
>> function it
> appears that I can use the name "Args" to refer to the type of the
> argument pack, and maybe in the return type as well?  But this isn't
> documented, so can I rely on it?

Apparently not, some more experimentation suggests that "Args" can't be 
easily used to compute the return type; if I use:

typename compute_result<Args>::type

as the return type of the function in BOOST_PARAMETER_FUNCTION, then 
everything is OK, provided I don't try and actually extract anything from 
the type Args inside compute_result, but if I define it as:

template <class Args>
struct compute_result
{
   typedef typename boost::parameter::value_type<
     Args, tag::domain_error, throw_on_error_type>
        ::type type;
};

Then I get endless compiler errors starting with:

c:\data\boost\release\boost\boost\parameter\value_type.hpp(62) : error 
C2661: 'boost::mpl::assert_not_arg' : no overloaded function takes 2 
arguments
c:\data\boost\boost\sandbox\math_toolkit\libs\math\ide\scrap.cpp(83) : see 
reference to class template instantiation 
'boost::parameter::value_type<Parameters,Keyword,Default>' being compiled
with
[
Parameters=boost::mpl::pair<boost::parameter::aux::empty_arg_list,boost::parameter::void_>,
Keyword=tag::domain_error,
Default=throw_on_error_type
]
c:\data\boost\boost\sandbox\math_toolkit\libs\math\ide\scrap.cpp(101) : see 
reference to class template instantiation 'compute_result<Args>' being 
compiled
with
[
Args=boost::mpl::pair<boost::parameter::aux::empty_arg_list,boost::parameter::void_>
]
c:\data\boost\boost\sandbox\math_toolkit\libs\math\ide\scrap.cpp(101) : see 
reference to class template instantiation 
'boost_param_result_101policy<Args>' being compiled
with
[
Args=boost::mpl::pair<boost::parameter::aux::empty_arg_list,boost::parameter::void_>
]
c:\data\boost\release\boost\boost\parameter\value_type.hpp(62) : error 
C2866: 
'boost::parameter::value_type<Parameters,Keyword,Default>::mpl_assertion_in_line_67' 
: a const static data member of a managed type must be initialized at the 
point of declaration
with
[
Parameters=boost::mpl::pair<boost::parameter::aux::empty_arg_list,boost::parameter::void_>,
Keyword=tag::domain_error,
Default=throw_on_error_type
]
c:\data\boost\boost\sandbox\math_toolkit\libs\math\ide\scrap.cpp(101) : 
error C2146: syntax error : missing ';' before identifier 'type'
c:\data\boost\boost\sandbox\math_toolkit\libs\math\ide\scrap.cpp(101) : 
error C3254: 'boost_param_result_101policy<Args>' : class contains explicit 
override 'type' but does not derive from an interface that contains the 
function declaration
with
[
Args=boost::mpl::pair<boost::parameter::aux::empty_arg_list,boost::parameter::void_>
]
c:\data\boost\boost\sandbox\math_toolkit\libs\math\ide\scrap.cpp(101) : 
error C2838: 'type' : illegal qualified name in member declaration
c:\data\boost\boost\sandbox\math_toolkit\libs\math\ide\scrap.cpp(101) : 
error C4430: missing type specifier - int assumed. Note: C++ does not 
support default-int

Obviously it's not easy to figure out what's going on here, given that this 
happens inside a macro expansion :-(

Thanks for any insight, John.