A trick using #if pre-processor

I hit a small C++ grammar trick which I have never met before in my project. The problem is how to deal with the situation that two defined identifiers have the same compilation behavior. A simple solution to this is to write two same block code like this:

#ifdef  identifier1
#ifdef  identifier2
// do something thing
#endif
#endif

Doing this seems a little odd , and I finally write the code like below:

#if defined(identifier1) && defined(identifier2)
// do something
#endif

Similarly, if the situation is #ifndef, it can be written as below:

#if !defined(identifier1) && !defined(identifier2)
// do something
#endif

However the defined directive can only be used in an #if and an #elif directive, but nowhere else. The following code block shows an example like a “if…else if… else…” flow chart.

#if defined(ALICE)
alice();
#elif defined(BOB)
bob();
#else
printerror();
#endif

The function call to alice is compiled if the identifier ALICE is defined. If the identifier BOB is defined, the function call to bob is compiled. If neither identifier is defined, the call to printerror is compiled.