Unfamiliar defines in config

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CPP, the C/C++ Preprocessor

CPP is the C & C++ Preprocessor. When you compile C or C++, instead of just reading the file from disk, the compiler asks CPP to read the file. CPP is designed to operate on a stream of text. It reads one line at a time, does whatever operations it needs to do, and then writes that line out to the compiler. To CPP, the lines below the current line is the future, and it has no clue about what will happen in the future (until it gets there). Most preprocessor things are done with preprocessor statements, which are lines which start with a #. These lines tell the preprocessor what to do, and are not part of the output that is sent to the compiler. comments When CPP encounters a comment, it removes it. However, it keeps the newlines so that line numbers will still be accurate. Comments come in two different forms. The first one begins with /* and ends with */. The other begins with // and ends at the end of the line. // This is a comment (which cannot be longer than one line) // (unless you have another comment right below...) /* this is also a comment but this comment continues over several lines */ #if / #endif #if works a lot like a comment, but it depends on the value of a condition. Both of these examples will output nothing: /* this will get removed */ #if 0 this will also get removed #endif #if 1 this will not be removed #endif expressions After #if you can put an expression, which gets evaluated using integer (whole-number) math. You can use the following operators: + - * / ~ ! % ^ & == != < > <= >= ( ) If the result is 0, the code inside the if is removed, if it is anything else, it is not removed. Expressions can also contain defined(SOME_DEFINED) which evaluates to 1 if the SOME_DEFINE is defined and 0 otherwise. #define This creates a new define. Defines works like a dictionary, translating one word into something else. It’s basically a search-and-replace for the source code. Defines can be used in different ways. Sometimes all we care about is if the define exists or not, like this: #define HAVE_TEA #if defined(HAVE_TEA) drink_tea(); #endif In this case HAVE_TEA is added to the dictionary, but it doesn’t expand to anything, it also doesn’t matter what it expands to, because the #if only checks if it’s in the dictionary or not. #define TEA_FLAVOR "Earl Grey" drink_tea(TEA_FLAVOR); In this example, TEA_FLAVOR expands to "Earl Grey", so the final output becomes: drink_tea("Earl Grey"); Defines can also hav…