Preprocessor question related to C2121 - '#' : invalid character : possibly the result of a macro expansion

[Gustavo L. Fabro]

Greetings.

Is there a way to run the preprocessor twice? Rephrasing that, is there a
way to:

#define SOMETHING #pragma OTHERTHING

and have the preprocessor in somecode.cpp evaluate:

SOMETHING

as

#pragma OTHERTHING

and then evaluate this #pragma directive as it should?

I know the C++ standard under 16.3.4 says:

"The resulting completely macro replaced preprocessing token sequence is not
processed as a preprocessing directive even if it resembles one."

but the C standard (1999) under 6.10.3.4 appears to change this to allow
#pragma directives in macros:

"The resulting completely macro-replaced preprocessing token sequence is not
processed as a preprocessing directive even if it resembles one, but all
pragma unary operator expressions within it are then processed as specified
in 6.10.9 below."

I know C is not C++ and so MSVC shouldn't actually allow this over my C++
code (if I'm interpreting the standard right). But is there a way to do it?
;-)

In case you want to know why I want this: I'm performance tuning my
application and I have a couple of functions and classes I'd like to compile
as native code.

Some of these classes have methods that call .NET Framework functions. I
have to explicity set these particular methods with a #pragma managed so
they will compile, Interop will do its job and it will run.

My code is being compiled under 2 different compilers. I'm using #define and
#ifdef directives to accomplish this. And my other compiler keeps shooting
me warnings over unknows #pragmas (such as #pragma unmanaged, for instance).

Having said that, I'd like to define things like:

#ifdef VISUAL_C //I #define this when using my MSVC compiler
#define SET_CLR_MODE #pragma managed
#define SET_NATIVE_MODE #pragma unmanaged
#else
#define SET_CLR_MODE
#define SET_NATIVE_MODE
#endif

and just SET_CLR_MODE whenever I needed IL and SET_NATIVE_MODE when I wanted
to go back to native.
Is it possible?

Thanx,

Fabro

[Doug Harrison [MVP]]

On Thu, 12 May 2005 17:49:15 -0300, Gustavo L. Fabro wrote:

I know the C++ standard under 16.3.4 says:

"The resulting completely macro replaced preprocessing token sequence is not
processed as a preprocessing directive even if it resembles one."
<snip>
but the C standard (1999) under 6.10.3.4 appears to change this to allow
#pragma directives in macros:
<snip>
I know C is not C++ and so MSVC shouldn't actually allow this over my C++
code (if I'm interpreting the standard right). But is there a way to do it?
<snip>
Having said that, I'd like to define things like:

#ifdef VISUAL_C //I #define this when using my MSVC compiler
#define SET_CLR_MODE #pragma managed
#define SET_NATIVE_MODE #pragma unmanaged
#else
#define SET_CLR_MODE
#define SET_NATIVE_MODE
#endif

and just SET_CLR_MODE whenever I needed IL and SET_NATIVE_MODE when I wanted
to go back to native.
Is it possible?

Offhand, I don't know the answer to your question, but if it turns out you
can't directly accomplish it, you could fall back on the method used by the
"pshpackN.h " and "poppack.h" header files. That is, you would define header
files such as:

gomanaged.h:

#ifdef VISUAL_C
#pragma managed
#endif

gounmanaged.h:

#ifdef VISUAL_C
#pragma unmanaged
#endif

Then you would #include these files instead of using the #pragmas directly.

--
Doug Harrison
Microsoft MVP - Visual C++

[Gustavo L. Fabro]

> Offhand, I don't know the answer to your question, but if it turns out you

can't directly accomplish it, you could fall back on the method used by
the
"pshpackN.h " and "poppack.h" header files. That is, you would define
header
files such as:

gomanaged.h:

#ifdef VISUAL_C
#pragma managed
#endif

gounmanaged.h:

#ifdef VISUAL_C
#pragma unmanaged
#endif

Then you would #include these files instead of using the #pragmas
directly.

Hi there!

Indeed that was my first plan. I just though it would be a bit nasty to have
#include directives in the middle of my function implementations .

I decided to google that particular piece of the standard and found several
messages on the subject. Guess it's really impossible to do it in the
#define way I had planned.

But thank you for the reply, anyway! :)

Fabro