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(cpp-295.info)Conditionals

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Conditionals
============

   In a macro processor, a "conditional" is a directive that allows a
part of the program to be ignored during compilation, on some
conditions.  In the C preprocessor, a conditional can test either an
arithmetic expression or whether a name is defined as a macro.

   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.

Uses

Conditional Uses. What conditionals are for.

Syntax

Conditional Syntax. How conditionals are written.

Deletion

Deleted Code. Making code into a comment.

Macros

Conditionals-Macros. Why conditionals are used with macros.

Assertions

How and why to use assertions.

Errors

#error Directive. Detecting inconsistent compilation parameters.