Re: [Boost-users] [parameter] Error using positional arguments

From: David Abrahams (dave_at_[hidden])
Date: 2006-01-30 18:49:39

Cromwell Enage <sponage_at_[hidden]> writes:

> --- David Abrahams wrote:
>> I take it back; I'm pretty sure this has nothing to
>> do with forwarding.  On what line of *your code*
>> does that error appear?
>
> The error appears on the line with the // error
> comment.

The actual error, and not just some line of the instantiation
backtrace?  That makes no sense to me, because the functions generated
by BOOST_PARAMETER_FUN take their arguments by const&.  The error
should appear wherever you try to bind a non-const reference to the
resulting const lvalue.  Aha:

    typedef typename boost::remove_const<
                            ^^^^^^^^^^^^
                typename boost::parameter::binding<
                    Params
                  , tag::rng_engine
                >::type
            >::type
            RNGEngine;

    ...

    RNGEngine rng_engine = p[example::rng_engine];

According to
http://www.boost.org/libs/parameter/doc/html/reference.html#binding, 
binding<Params,Tag>::type is always a reference type.  Since

   remove_const<T&> is T&

when T is const (as it is in this case), you get a reference to
const.  Thus RNGEngine is 

  boost::mt19937 const&

and the error should come here:

    typedef boost::variate_generator<RNGEngine,RNGDist> RNG;
    ...
    RNG rng(rng_engine, RNGDist(p[min], p[max]));

where variate_generator takes its first constructor argument by
mutable reference.

If my analysis is wrong, please explain how it is so.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com