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

2 Oct 2007

      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