Re: [Boost-users] boost.parameter - best practices

13 Apr 2011

      At Wed, 13 Apr 2011 07:50:52 +0200,
Christoph Heindl wrote:
...
Hi,
I have a set of classes that offer named parameter enabled
constructors through the use of argument packs. Currently I have one
huge header container all the available arguments of all classes.
Wow.  We designed this library so that you could avoid doing anything
like that!  That causes a lot of compile-time coupling: every time you
add a new keyword, you recompile the world.
...
// params.h
namespace geo {
  namespace keywords {
    BOOST_PARAMETER_NAME(a)
    BOOST_PARAMETER_NAME(b)
  }
}
// thing.h
namespace geo {
  class thing {
  public:
    template<class Opts>
    thing(const Opts &o) {
     _m_x = o[Keywords::_a | 10];
     //etc..
    }
  };
}
// thing2.h
namespace geo {
  class thing2 {
  public:
    template<class Opts>
    thing2(const Opts &o) {
     _m_x = o[Keywords::_b | 10];
     //etc..
    }
  };
}
First question: I'd like to place the BOOST_PARAMETER_NAME keywords as
close to its usage as possible. I.e. I'd like to place
BOOST_PARAMETER_NAME(a) on top of thing.h. If I do so, I think that
over time other classes will start to have their own
BOOST_PARAMETER_NAME(a) constructs in their header files, which I
think would lead to multiple defined symbols if two class headers are
included in the same translation unit, correct?
I think so.  I think there may be a cure for this that we never got
around to implementing in the library.  If you want to make a small
reproducible test case and open a Trac ticket, there's a good chance
we can address it.
...
So, is their any way to declare a parameter inside a class, through a
typedef or similar?
I don't know.  Did you try just using BOOST_PARAMETER_NAME at class
scope?  (I'm pretty sure that won't work but it's worth a shot).
...
Let me illustrate
class x {
public:
  typedef boost::parameter::keyword<...> a;
template<class Opts>
  x(const Opts &o) {
   _m_x = o[a | 10];
If "a" is a type, the above is illegal syntax (imagine substituting
"int" there).
...
}
};
x my_x(x::a = 5);
If that is not possible, is there any way to limit the template
parameter Opts in the above example to set of possible parameters?
Do you mean, any way to limit which keywords are allowed?  

Sure.  Oh, wow, I see that's been dropped from the documentation!  Yikes!
And I can't reconstruct it from memory (double Yikes)!

Daniel, can you try to explain how to use
boost::parameter::parameters<...>::type to do SFINAE?

(Sorry I couldn't help further)

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com