You can define a flag on the command line by using the –D switch. So, to define the myway flag, you would use the following: -D myway.

Then, you use it in the following way:

#if myway
//Code here compiled only if myway is defined
#else
//Code here compiled in the other case
#end

There is also a not operator:

#if !myway
//Code here compiled only myway is not defined
#end

Conditional compilation depending on the target basically works in the same way, except that the name of the target you are compiling to automatically gets set. So, you will have something like the following:

#if cpp
//C++ code...