how to make a macro stringify and paste a -D defined arg

[g++ 4.4.0, linux, large SW project so I can't change compilers or the
api I'm talking to]
I have a macro:
#define RegisterFactory(clazz) \
clazz clazz##_instance; \
extern "C" Foo::HHFactory *get##clazz() { return
&clazz##_instance; } \
Foo::HHRegistrar clazz##_registrar(#clazz)
which works fine for literal arguments. But when I give it an
argument that was defined on the compile line:

g++ -DFACTORYNAME=MyFactory

it fails. I understand why... the arg is used in paste and stringify
contexts -- but is there some reasonable way to fix this?
I want
RegisterFactory(FACTORYNAME) to expand to:
MyFactory MyFactory_instance;
extern "C" Foo::HHFactory *getMyFactory() { return
&MyFactory_instance; } \
Foo::HHRegistrar MyFactory_registrar("MyFactory")

The original macro is used in many places so I need something that
cleanly replaces it. I tried #define FACTORYNAME MyFactory, but no
difference(not that I really expected one).

gry <georgeryoung@gmail.com> writes:

> #define RegisterFactory(clazz) \
> clazz clazz##_instance; \
> extern "C" Foo::HHFactory *get##clazz() { return &clazz##_instance; } \
> Foo::HHRegistrar clazz##_registrar(#clazz)
> which works fine for literal arguments. But when I give it an
> argument that was defined on the compile line:
>
> g++ -DFACTORYNAME=MyFactory
>
> it fails.

Define a new macro that takes FACTORYNAME as an argument and
forwards it to your original macro (renamed below):

#define RegisterFactoryNoExpand(clazz) \
clazz clazz##_instance; \
extern "C" Foo::HHFactory *get##clazz() { return &clazz##_instance; } \
Foo::HHRegistrar clazz##_registrar(#clazz)
#define RegisterFactory(clazz) RegisterFactoryNoExpand(clazz)

#define FACTORYNAME MyFactory
RegisterFactory(FACTORYNAME);

Then, the preprocessor will expand FACTORYNAME.
http://c-faq.com/ansi/stringize.html

On Apr 14, 2:25*am, Kalle Olavi Niemitalo <k...@iki.fi> wrote:
> gry <georgeryo...@gmail.com> writes:
> > #define RegisterFactory(clazz) \
> > * * clazz clazz##_instance; \
> > * * extern "C" Foo::HHFactory *get##clazz() { return &clazz##_instance; } \
> > * * Foo::HHRegistrar clazz##_registrar(#clazz)
> > which works fine for literal arguments. *But when I give it an
> > argument that was defined on the compile line:
>
> > g++ -DFACTORYNAME=MyFactory
>
> > it fails.
>
> Define a new macro that takes FACTORYNAME as an argument and
> forwards it to your original macro (renamed below):
>
> #define RegisterFactoryNoExpand(clazz) \
> * * clazz clazz##_instance; \
> * * extern "C" Foo::HHFactory *get##clazz() { return &clazz##_instance; } \
> * * Foo::HHRegistrar clazz##_registrar(#clazz)
> #define RegisterFactory(clazz) RegisterFactoryNoExpand(clazz)
>
> #define FACTORYNAME MyFactory
> RegisterFactory(FACTORYNAME);
>
> Then, the preprocessor will expand FACTORYNAME.http://c-faq.com/ansi/stringize.html

That works beautifully. Thanks!