Re: [Boost-users] [pre-processor ] help needed

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2008-01-28 14:08:39

Steven,

thanks a lot for your answers. That could help, but I would like to have
some simple interface for the user. I don't want users write a lot of
template classes where they would not really understand the background.

In the current approach X4 is the trait type:

Asking the users to write:

(X1)(X2)(X3)((traitX<T1)(T2>)) is not a great idea.

As well as:
(X1)(X2)(X3)(paranthesized_type(wrap<traitX<T1, T2> >))

I could try doing it in the background and still ask users write:

(X1)(X2)(X3)((traitX<T1,T2>))

I just thought there could be a simpler way of doing it.

Many thanks for your help,

Ovanes

On Jan 28, 2008 7:12 PM, Steven Watanabe <watanabesj_at_[hidden]> wrote:

> AMDG
>
> Ovanes Markarian wrote:
> > Hello *,
> >
> >
> > I have some pre-processor sequence in form of:
> >
> > (X1) (X2) (X3) (X4)
> >
> > which is expanded by BOOST_PP_SEQ_FOR_EACH to template parameters:
> >
> > some_template<X1, X2, X3, X4> ...
> >
> >
> > My problem now is that some of Xn parameters can be a template
> > instantiation and that actually causes PP to think, that there are
> > more parameters to the function applied by ..._SEQ_FOR_EACH.
> > Example: some_other_template<T1, T2> cause preprocessor to think that
> > there are 2 params some_other_template<T1 and T2>.
> >
> > Putting the parameter into additional braces like ((X4)) results in
> > the template:
> >
> > some_template<X1, X2, X3, (X4)> and that results in a compiler error.
> > I know this construct can switch off ADL, but I don't know why it
> > result here in a compiler error, since I fully qualify the X4 (from
> > which namespace it is). Is there any way to treat Xn as a single
> > paramer but during the final pre-processing stage get rid of
> > additional braces?
> >
>
> There are a couple of ways to do it.  You can make each element of the
> Seq to be itself a seq
> and then call BOOST_PP_SEQ_ENUM on the inside:
>
> ((X1))  ((X2))  ((some_other_template<T1)(T2>))
>
> Another way is to use template metaprogramming
>
> (X1)(X2)(parenthesize_type(wrap<some_other_template<T1, T2>>))
>
> template<class T>
> struct unparenthesize { typedef T type; };
>
> template<class T>
> struct unparenthesize<parenthesized_type(wrap<T>)> { typedef T type; };
>
> In Christ,
> Steven Watanabe
>
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