C preprocessor: Difference between revisions

The preprocessor can ''define'' tokens. It is good custom to write these tokens in ALL CAPS. (See pt. 2 as for why.)

 

#define MYTOKEN

 

Most compilers also allow the definition of preprocessor tokens on the command line, e.g. the "-D MYTOKEN" option for [[GCC]].

The preprocessor can ''conditionally'' select which parts of source code to compile, depending on whether a given token is defined or not (see above).

 

#define MYTOKEN

 

/* This source will be compiled */

#endif

 

Note that such ''#if'' / ''#ifdef'' / ''#ifndef'' - ''#endif'' sections can be nested.

The solution are ''header guards'', a combination of conditional compilation and token definition:

 

/* abc.h */

 

 

#endif

 

== Preprocessor Macros, pt. 2 ==

The ''#if'' statement can be used to base conditional compilation on token values. Note that the preprocessor can only work with compile-time constants. Compiler-evaluated code like `sizeof()` cannot be used in preprocessor directives. On the upside, the preprocessor can natively handle non-numerical values.

 

#define MYTOKEN foo

#define OTHERTOKEN 42

/* Won't be compiled. */

#endif

 

The ''#if'' directive also allows for a simple construct to disable a region of code without having to worry about nested ''/* ... */'' style comments:

 

#if 0

/* disabled code */

#endif

 

Such code can easily be re-enabled temporarily with no more effort than replacing the "0" with a "1". Source comments as to why you disabled code this way are in order.

For production code, assertions may be turned off by defining NDEBUG:

 

gcc -DNDEBUG ...

 

Note that <assert.h> does not have (or need) a header guard, i.e. can be included multiple times in a source file, and that whether NDEBUG is defined or not is evaluated anew ''at every inclusion of <assert.h>''. You can thus enable / disable assertions at a very fine-grained level if necessary: