Re: [Boost-users] [lambda] extract a constant subexpression by introspection

Subject: Re: [Boost-users] [lambda] extract a constant subexpression by introspection
From: alfC (alfredo.correa_at_[hidden])
Date: 2010-05-26 01:07:24

On May 21, 6:21 pm, alfC <alfredo.cor..._at_[hidden]> wrote:
>  I have a lambda expression that is suppoalways constructed as
>    [some lambda expression] / ( _1 - c);
> What I need is a function that given the expression returns the
> internal value of c, if that is possible:

Answering to myself, hope it is useful to someone else:

It turns out that extracting c from the lambda expression f_expr is
quite easy. just
        boost::tuples::get<1>(boost::tuples::get<1>(f_expr.args).args),

The tricky part is to write the argument type of the function that
extracts 'c' only in the case that the expression if of type 'XX/(_1-
c)'
Below is the full function that has to be defined to extract c, if the
expression is not of type XX/(_1-c) the function is not even
instantiated:

        template<class LambdaExp>
        double extract_c(
                //typename simple_rational_function<LambdaExp>::type const&  //
doing a convenient typedef doesn't work, at least for gcc
                lambda_functor<lambda_functor_base<
                                arithmetic_action<divide_action>,
                                tuple<
                                        lambda_functor<//
                                                LambdaExp
					>,//
                                        lambda_functor<lambda_functor_base<
                                                        arithmetic_action<minus_action>,
                                                        tuple<
                                                                lambda_functor<
                                                                        placeholder<1>
								>,
                                                                double const // <-- attention adds const to avoid unreadable
errors
							>
					> >
				>
		> >
                        f_expr){
                     return
boost::tuples::get<1>(boost::tuples::get<1>(f_expr.args).args);
              }

Suggestions accepted,
Alfredo