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A conditional is a preprocessing directive that controls whether
or not to include a chunk of code in the final token stream that is
compiled. Preprocessing conditionals can test arithmetic expressions,
or whether a name is defined as a macro, or both together using the
special defined operator.
A preprocessing conditional in C 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.
Sometimes this distinction makes no practical difference. GCC and
other 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 that
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 file
remains syntactically valid when that code is not used.
Next: Diagnostics, Previous: Macros, Up: Preprocessing [Contents][Index]