Preprocessor magic needed

[claus.tondering@gmail.com]

I need to write a macro that inserts
someStruct m_someStruct;
into another struct declaration.

The problem is that if the programmer specifies one particluar struct
(called alpha), nothing should be inserted.

So, is it possible to write a macro that does this:

MACRO(alpha) expands to nothing
MACRO(X) expands to X m_##X (for all values of X except alpha)


My guess is that it is impossible. But then, in the Boost preprocessor
package, they achieve some things that I would also have thought
impossible. :-)

This has to work in C, so I cannot use C++ templates.

--
Claus Tondering

[Eric Sosman]

Re: Preprocessor magic needed



claus.tondering @gmail.com wrote On 03/17/06 09:26,:[color=blue]
> I need to write a macro that inserts
> someStruct m_someStruct;
> into another struct declaration.
>
> The problem is that if the programmer specifies one particluar struct
> (called alpha), nothing should be inserted.
>
> So, is it possible to write a macro that does this:
>
> MACRO(alpha) expands to nothing
> MACRO(X) expands to X m_##X (for all values of X except alpha)[/color]

The expansion of a macro cannot produce preprocessor
directives, hence it cannot produce preprocessor conditionals.

However, you might be able to do something like

#define alpha /* nil */
#define m_alpha /* nil */

.... so that when MACRO(alpha) expands, each component of its
expansion is also a macro that then "expands" to nothing.

Personally, I don't think the preprocessor is the right
tool for this.
[color=blue]
> This has to work in C, so I cannot use C++ templates.[/color]

Why cross-post to comp.lang.c++, then? Followups
set to comp.lang.c only.

--
Eric.Sosman@sun .com

[Paul Mensonides]

Re: Preprocessor magic needed

claus.tondering @gmail.com wrote:[color=blue]
> I need to write a macro that inserts
> someStruct m_someStruct;
> into another struct declaration.
>
> The problem is that if the programmer specifies one particluar struct
> (called alpha), nothing should be inserted.
>
> So, is it possible to write a macro that does this:
>
> MACRO(alpha) expands to nothing
> MACRO(X) expands to X m_##X (for all values of X except alpha)[/color]

Yes, it is possible:

#include <boost/preprocessor/cat.hpp>
#include <boost/preprocessor/control/expr_iif.hpp>
#include <boost/preprocessor/detail/is_nullary.hpp>
#include <boost/preprocessor/logical/compl.hpp>

#define LIBRARY_MACRO(i d) \
BOOST_PP_EXPR_I IF( \
BOOST_PP_COMPL( \
BOOST_PP_IS_NUL LARY( \
BOOST_PP_CAT(LI BRARY_MACRO_, id) \
) \
), \
X BOOST_PP_CAT(m_ , X) \
) \
/**/
#define LIBRARY_MACRO_a lpha ()

LIBRARY_MACRO(a lpha) // expands to nothing
LIBRARY_MACRO(X ) // expands to X m_X

Regards,
Paul Mensonides

[Bob Hairgrove]

Re: Preprocessor magic needed

On Sat, 18 Mar 2006 00:53:06 -0800, "Paul Mensonides"
<pmenso57@comca st.net> wrote:
[color=blue]
>claus.tonderin g@gmail.com wrote:[color=green]
>> I need to write a macro that inserts
>> someStruct m_someStruct;
>> into another struct declaration.
>>
>> The problem is that if the programmer specifies one particluar struct
>> (called alpha), nothing should be inserted.
>>
>> So, is it possible to write a macro that does this:
>>
>> MACRO(alpha) expands to nothing
>> MACRO(X) expands to X m_##X (for all values of X except alpha)[/color]
>
>Yes, it is possible:
>
>#include <boost/preprocessor/cat.hpp>
>#include <boost/preprocessor/control/expr_iif.hpp>
>#include <boost/preprocessor/detail/is_nullary.hpp>
>#include <boost/preprocessor/logical/compl.hpp>
>
>#define LIBRARY_MACRO(i d) \
> BOOST_PP_EXPR_I IF( \
> BOOST_PP_COMPL( \
> BOOST_PP_IS_NUL LARY( \
> BOOST_PP_CAT(LI BRARY_MACRO_, id) \
> ) \
> ), \
> X BOOST_PP_CAT(m_ , X) \
> ) \
> /**/
>#define LIBRARY_MACRO_a lpha ()
>
>LIBRARY_MACRO( alpha) // expands to nothing
>LIBRARY_MACRO( X) // expands to X m_X
>
>Regards,
>Paul Mensonides
>[/color]

Hmmm ... he said it needs to work in C ... does Boost compile as C
code?

--
Bob Hairgrove
NoSpamPlease@Ho me.com

[Paul Mensonides]

Re: Preprocessor magic needed

Bob Hairgrove wrote:[color=blue]
> On Sat, 18 Mar 2006 00:53:06 -0800, "Paul Mensonides"[/color]
[color=blue][color=green]
>> Regards,
>> Paul Mensonides
>>[/color]
>
> Hmmm ... he said it needs to work in C ... does Boost compile as C
> code?[/color]

The pp-lib is both C and C++ code. The rest of Boost is not.

Regards,
Paul Mensonides

[claus.tondering@gmail.com]

Re: Preprocessor magic needed

With the small change that the X in your macro should be replaced by
id, it works beautifully! Thank you very much.

--
Claus Tondering

[claus.tondering@gmail.com]

Re: Preprocessor magic needed

With the small change that the X in your macro should be replaced by
id, it works beautifully! Thank you very much.

--
Claus Tondering

[claus.tondering@gmail.com]

Re: Preprocessor magic needed

With the small change that the X in your macro should be replaced by
id, it works beautifully! Thank you very much.

--
Claus Tondering

[CBFalconer]

Re: Preprocessor magic needed

claus.tondering @gmail.com wrote:[color=blue]
>
> With the small change that the X in your macro should be replaced
> by id, it works beautifully! Thank you very much.[/color]

I suppose we should be thankful that such an incomprehensibl e and
useless context free message, which was even stupidly cross-posted
to c.l.c++ and c.l.c, is short.

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