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Oct 7th, 2008

Preprocessor #warning Command Question

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Is there a way to #define something as a #warning?
I tried using the following code, but it gives me an error:
'warning' undeclared (first use this function)

This is the part of code where the error appears:

c++ Syntax (Toggle Plain Text)
#define something #warning This is not a standard function!
 
int main(void)
{
something
return 0;
}
#define something #warning This is not a standard function!

int main(void)
{
something
return 0;
}

Any help on this please?
If it's not possible to perform, please tell

Thanks in advance

Last edited by unbeatable0; Oct 7th, 2008 at 10:45 am.

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Oct 7th, 2008

Re: Preprocessor #warning Command Question

Never mind, I found the answer

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Oct 7th, 2008

Re: Preprocessor #warning Command Question

Oh well... I was wrong. My method didn't work

Please help me

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Oct 7th, 2008

Re: Preprocessor #warning Command Question

I don't think the pre-processor is capable of expanding lines which are themselves pre-processing statements.

In short, I don't think it's going to work.

Salem

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Oct 8th, 2008

Re: Preprocessor #warning Command Question

Any help on this? If it helps, I want the macro to be a function. For example:

c++ Syntax (Toggle Plain Text)
#ifndef _WINDOWS_H
#define test(); #warning <windows.h> needs to be included for this function!
#endif
int main(void)
{
test();
return 0;
}
#ifndef _WINDOWS_H
#define test(); #warning <windows.h> needs to be included for this function!
#endif
int main(void)
{
test();
return 0;
}

If it's not possible to do this way, is there another way to do it, which will give a warning instead of the function, in case the function cannot be used?

Last edited by unbeatable0; Oct 8th, 2008 at 4:46 am.

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Oct 8th, 2008

Re: Preprocessor #warning Command Question

Salem is right; there is no way to #define a macro that expands to a preprocessor directive.

There is also the incidental concern that #warning is not a standard preprocessor directive.

Any C++ compiler will generate an error on an attempt to call a function that has not previously been declared.

I can't see the point of what you're trying to do. If <window.h> is not included

C++ Syntax (Toggle Plain Text)
#ifndef _WINDOWS_H
#error <windows.h> needs to be included to compile this source file
#endif
#ifndef _WINDOWS_H
#error <windows.h> needs to be included to compile this source file
#endif

will generate an error message that gives specific information on requirements.

Of course, if you need to use the above (eg if you have a header that relies on <windows.h>) it would be easier to #include <windows.h> anyway

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Oct 8th, 2008

Re: Preprocessor #warning Command Question

> #define test(); #warning <windows.h> needs to be included for this function!
But if you don't have windows.h, then the code would have failed to compile before this point anyway.

How to tell your OS/Compiler/etc at compile-time
http://predef.sourceforge.net/

You have to do something like this

C++ Syntax (Toggle Plain Text)
#ifdef _WIN32
#include <windows.h>
#else
#warning needs windows.h
// in case the compiler ignores #warning
int test ( ) {
  fprintf( stderr, "_WIN32 not set, compilation should have failed!\n" );
  exit( 0 );
  return 0;
}
#endif
#ifdef _WIN32
#include <windows.h>
#else
#warning needs windows.h
// in case the compiler ignores #warning
int test ( ) {
  fprintf( stderr, "_WIN32 not set, compilation should have failed!\n" );
  exit( 0 );
  return 0;
}
#endif

Or if you're trying to spot obsolete functionality, and you're using gcc, then perhaps this (this is the 2nd obscure snippet from the gcc manual today

)

Quote originally posted by gcc manual ...

deprecated
The deprecated attribute results in a warning if the variable is used anywhere
in the source file. This is useful when identifying variables that are expected
to be removed in a future version of a program. The warning also includes the
location of the declaration of the deprecated variable, to enable users to easily
find further information about why the variable is deprecated, or what they
should do instead. Note that the warning only occurs for uses:
extern int old_var __attribute__ ((deprecated));
extern int old_var;
int new_fn () { return old_var; }
results in a warning on line 3 but not line 2.

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