Using define's to turn on/off functionality

[daviddoria]

I would like to have a global way to do something like this

#ifdef GRAPHICS
//do some stuff that relies on graphics libraries that may or may not be installed
#endif

The problem with defines is that they have to be defined in every file, or at least be defined in a header that is included in every file that I wish to do this.

What I'm looking for is more of maybe a compiler option or something? Where I can say

g++ MyProgram +GRAPHICS
or
g++ MyProgram -GRAPHICS

And it will just ignore parts of the code completely if GRAPHICS is not specified.

Sorry if this is unclear, but I'll explain as we go! haha

Thanks,
Dave

[Duoas]

That is typically something handled by the build process.

If you want to have the #defines in every file, you can specify the flag, or not, in your Makefile(s).

Here's an example makefile that builds a program "foo" from the following files:
foo.cpp
baz.h
baz.cpp
quux.h
quux.cpp

If compiling on Windows, it will automatically #define GRAPHICS for every source file and link the executable with the Windows GDI library.

#----------------------------------------
# Common stuff
#
CC     = g++
CFLAGS = -Wall
LFLAGS = 

#----------------------------------------
# Windows-specific nonsense
#
Windows = $(if $(COMSPEC)$(ComSpec),1,)

ifeq ($(Windows),1)
CFLAGS = $(CFLAGS) -DGRAPHICS
LFLAGS = $(LFLAGS) -lgdi32
endif

#----------------------------------------
# Main target rule
#
foo: foo.o baz.o quux.o
	$(CC) -o foo $^ $(LFLAGS)

#----------------------------------------
# Individual object files rule
#
%.o: %.cpp %.h
	$(CC) $(CFLAGS) -c $<

Another common option is to have a file (often named something like "machine.cpp") for each type of architecture that actually implements the OS-dependant code, and a common "machine.h" file which prototypes the routines implemented by the machine files. That way your program can use the machine.h API to do OS-dependant stuff. And again, the build system will compile and link with the appropriate version of the "machine.cpp" file.

Hope this helps get you going.

[daviddoria]

ah great! so

g++ -DGRAPHICS

is the same as having all the files being compiled having a #define GRAPHICS 1 ?

so then I could even use
if(GRAPHICS)

instead of
#ifdef GRAPHICS

?

Thanks,

Dave

[Duoas]

No, it is the same as all the files having a #define GRAPHICS To have a value: g++ -DGRAPHICS=1 --> #define GRAPHICS 1 if (GRAPHICS) will work if and only if GRAPHICS is always defined to have some value (such as 0 or 1). That is typically a wasteful way to do it though, because you add time used and space occupied for every test. It is best to stick with the #ifdef GRAPHICS so that only the code you need gets compiled into the executable.

[edit]
You can still test for different values though:

#if GRAPHICS == 1
  do_x();
#elif GRAPHICS == 2
  do_y();
#else
  do_z();
#endif

Hope this helps.

[daviddoria]

fantastic ! exactly what I wanted.

Dave