Re: [Boost-users] [parameter] Return type dependent on parameter type?

From: Daniel Wallin (daniel_at_[hidden])
Date: 2007-10-02 15:28:27

François Duranleau wrote:
> On Mon, 1 Oct 2007, David Abrahams wrote:
> 
> [...]
>> See
>> http://boost.org/libs/parameter/doc/html/index.html#adding-type-requirements
> 
> I tried this simple example:
> 
> //---------------------------------------------
> #include <boost/parameter/name.hpp>
> #include <boost/parameter/preprocessor.hpp>
> 
> BOOST_PARAMETER_NAME( (arg1 , kw) arg1 )
> 
> BOOST_PARAMETER_FUNCTION(
>      (arg1_type) ,
>      the_function ,
>      kw ,
>      (required (arg1 , *))
> )
> {
>      return arg1;
> }
> 
> int
> main()
> {
>      int i = the_function( 1 ) ;
>      return 0 ;
> }
> //---------------------------------------------
> 
> but it won't compile either (gcc-4.0.2). Actually, if we look at the 
> preprocessed output, it's not surprising. Here is the part of interest 
[...]
> 
> I looked for examples in Boost.Parameter's test programs, alas, it seems 
> there are no tests written for this feature.

Right. This is a doc error. You need extract the type from the argument
pack Args:

 BOOST_PARAMETER_FUNCTION(
      (typename boost::parameter::value_type<Args, kw::arg1>::type) ,
      the_function ,
      kw ,
      (required (arg1 , *))
 )
 {
      return arg1;
 }

Unfortunately the library has a bug that prevents this from working.
This has been fixed, but not in time for 1.34. IIRC you need to apply
this patch:

http://svn.boost.org/trac/boost/attachment/ticket/1044/parameter.diff

for this to work correctly.

-- 
Daniel Wallin
Boost Consulting
www.boost-consulting.com