(cpp.info.gz) Conditionals

 
 4 Conditionals
 **************
 
 A "conditional" is a directive that instructs the preprocessor to
 select whether or not to include a chunk of code in the final token
 stream passed to the compiler.  Preprocessor conditionals can test
 arithmetic expressions, or whether a name is defined as a macro, or both
 simultaneously using the special `defined' operator.
 
    A conditional in the C preprocessor resembles in some ways an `if'
 statement in C, but it is important to understand the difference between
 them.  The condition in an `if' statement is tested during the
 execution of your program.  Its purpose is to allow your program to
 behave differently from run to run, depending on the data it is
 operating on.  The condition in a preprocessing conditional directive is
 tested when your program is compiled.  Its purpose is to allow different
 code to be included in the program depending on the situation at the
 time of compilation.
 
    However, the distinction is becoming less clear.  Modern compilers
 often do test `if' statements when a program is compiled, if their
 conditions are known not to vary at run time, and eliminate code which
 can never be executed.  If you can count on your compiler to do this,
 you may find that your program is more readable if you use `if'
 statements with constant conditions (perhaps determined by macros).  Of
 course, you can only use this to exclude code, not type definitions or
 other preprocessing directives, and you can only do it if the code
 remains syntactically valid when it is not to be used.
 
    GCC version 3 eliminates this kind of never-executed code even when
 not optimizing.  Older versions did it only when optimizing.
(cpp.info.gz) Conditionals

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