Preprocessor magic needed

[claus.tondering]

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]

cl*************@gmail.com wrote On 03/17/06 09:26,:

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

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

--
Er*********@sun.com

[Paul Mensonides]

cl*************@gmail.com wrote:

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)

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(id) \
BOOST_PP_EXPR_IIF( \
BOOST_PP_COMPL( \
BOOST_PP_IS_NULLARY( \
BOOST_PP_CAT(LIBRARY_MACRO_, id) \
) \
), \
X BOOST_PP_CAT(m_, X) \
) \
/**/
#define LIBRARY_MACRO_alpha ()

LIBRARY_MACRO(alpha) // expands to nothing
LIBRARY_MACRO(X) // expands to X m_X

Regards,
Paul Mensonides

[Bob Hairgrove]

On Sat, 18 Mar 2006 00:53:06 -0800, "Paul Mensonides"
<pm******@comcast.net> wrote:

cl*************@gmail.com wrote:

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)

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(id) \
BOOST_PP_EXPR_IIF( \
BOOST_PP_COMPL( \
BOOST_PP_IS_NULLARY( \
BOOST_PP_CAT(LIBRARY_MACRO_, id) \
) \
), \
X BOOST_PP_CAT(m_, X) \
) \
/**/
#define LIBRARY_MACRO_alpha ()

LIBRARY_MACRO(alpha) // expands to nothing
LIBRARY_MACRO(X) // expands to X m_X

Regards,
Paul Mensonides

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

--
Bob Hairgrove
No**********@Home.com

[Paul Mensonides]

Bob Hairgrove wrote:

On Sat, 18 Mar 2006 00:53:06 -0800, "Paul Mensonides"

Regards,
Paul Mensonides

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

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

Regards,
Paul Mensonides

[claus.tondering]

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]

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]

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]

cl*************@gmail.com wrote:

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

I suppose we should be thankful that such an incomprehensible 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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