1/16/2009

01-16-09 - push_macro & pop_macro

WTF MSVC has macro push & pop !? How did I not know this? It's so superior. It actually makes #defining new & delete actually possibly an okay option. (normally I get sucked into a hell of having to #undef them and redef them back to the right thing)


#define test 1

#pragma PRAGMA_MESSAGE( STRINGIZE(test) )

#pragma push_macro("test")

#undef test
#define test 2

#pragma PRAGMA_MESSAGE( STRINGIZE(test) )

#pragma pop_macro("test")

#pragma PRAGMA_MESSAGE( STRINGIZE(test) )

outputs : 1 , 2 , 1

Wow!

BTW this demo used these tricks :


#define _Stringize( L )            #L
#define _DoMacro1( M, X )        M(X)

#define STRINGIZE(M)            _DoMacro1( _Stringize, M )
#define LINE_STRING                STRINGIZE( __LINE__ )

#define PRAGMA_MESSAGE(str)        message( __FILE__ "(" LINE_STRING ") : message: " str)

Of course I can't use it at work where multi-platform support is important, but I can use it at home where I don't give a flying fuck about things that don't work in MSVC and it makes life much easier.

5 comments:

castano said...

GCC supports that as well:

http://gcc.gnu.org/onlinedocs/gcc/Push_002fPop-Macro-Pragmas.html

800poundgames said...

#define old_test test
#undef test
#define test whatever i want to do
#undef test
#define test old_test

nothings said...

Obviously what 800poundgames wrote doesn't work. There is a strategy to avoid the mess of knowing how to define it back to the original def, although you probably already know this:

#define THINGY_publicdef ...actual definition...
#define THINGY THINGY_publicdef

then in a place where you need to override it:

#undef THINGY
#define THINGY whatever

and then to undo it

#undef THINGY
#define THINGY THINGY_publicdef

cbloom said...

Yeah, Sean, obviously that is what I do, but that doesn't work when you don't know which publicdef to do, eg. when you're mixing codebases.

The most common case where I hit this is with malloc or new. I think I'll write more about that case, but the basic issue is :

#undef new

.. do stuff with original new

#define new my_new

but if this is like inside an STL header, it can't know to use the "my_new" from cblib, etc.

cbloom said...

BTW I wish you could pragmas inside #defines ; there's really no reason why not, #define is just text-sub.

old rants