#ifndef

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Controls conditional compilation of code based on the existence of an identifier assigned with #define.

Syntax

#ifndef <identifier>
<statements 1>

[#else
<statements 2>]

#endif

<identifier>

The identifier you want to test for. <identifier> is defined with the #define directive.

<statements 1>

Any number of statements and preprocessor directives. These lines are compiled if <identifier> has not been defined.

#else <statements 2>

Specifies the lines to compile if <identifier> has been defined.

Description

Use the #ifndef directive to conditionally compile sections of source code. If you haven’t defined <identifier> with #define, the code you specify with <statements 1> is compiled; otherwise, the code following #else, if any, is compiled.

Use #ifndef if you want to include code only if the identifier is not defined. Otherwise, you can use #ifdef to include code only if the identifier is defined, and #ifdef with its #else option to include different sets of code depending on the existence of the identifier.

You may nest conditional compilation directives.

Conditional compilation is useful when maintaining different versions of the same program, for debugging purposes, and for managing the use of #include files. Using #ifndef for conditional compilation is different than not executing code with an IF statement. With IF, the code still gets compiled into the resulting byte code file, even if it is never executed. By using #ifndef to exclude code you don’t want for a particular version of your program, the code is never compiled into byte code.