Re: macro

"Victor Bazarov" <>
Sat, 28 Feb 2009 11:45:04 -0500
asmcad wrote:

i think it's not about the compiler or operating system (??) note:
(i'm using gcc)

you can see a lots of samples on here :

but it seems stupid!
it going like this when i check

class WXSMTH classname ; //ok
#define WXSMTH WXSOMETHNELSE // say ok
#define WXSOMETHNELSE // what the heck is this!

what is the purpose?

The purpose is to keep the source code the same (the files that define
the classes) and make changes by either the command-line definition of
macros or by installing some additional headers that customize the
processing of the other headers.

If you're new to macros or conditional compilation, please get a good
book or join a decent project, and learn. Don't go around complaining
about something just because it's new to you.

                       * * *

A statement

  #define BLAH

in itself is very meaningful because after the preprocessor sees it,
the macro is *defined* and can be used in some special way in other
places while compiling the same translation unit. Now, what you
are seeing is slightly different, but still very useful.

If I write in some header <mydefinitions.h>

   #ifdef _UNIX
     #define SOMETHING short
     #define SOMETHING

and later I write in some C++ implementation file

   #include <mydefinitions.h>

   SOMETHING int myvariable;

Then when I compile that file on Unix, I would get

   short int myvariable;

but, when I compile that file on, say, Windows, I would get

   int myvariable;

(I am not going into *why* that might be useful, it's just the
example I chose to give). Now, imagine that instead of the ifdef
in the hedear <mydefinitions.h> I have a custom header that when
installed on Unix contains

    #define SOMETHING short

During the installation I rename the file 'mydefinitions.h'.
And on Windows I install another custom header (and rename it to
'mydefinitions.h'), which has

    #define SOMETHING

Now, I don't need to change the C++ source at all and it does
not need any conditional directives either. The difference
between the OSes is resolved during installation. Or it could
be resolved in the compiler command line where I tell it to get
the relevant headers from the OS-specific directory, and in
those directories there are files 'mydefinitions.h' with their
respective contents, but different. There are many variations
of the possible solution to whatever is needed.

Now, if you need to find out what the macro can resolve to on
other systems than yours, look in the same file after installing
your library for all other compilers. Or contact the library
publisher's Technical Support.

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