#ifdef

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

Syntax

#ifdef <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 been defined.

#else <statements 2>

Specifies the lines to compile if <identifier> has not been defined.

Description

Use the #ifdef directive to conditionally compile sections of source code. If you’ve defined <identifier> with #define, the code you specify with <statements 1> is compiled; otherwise, the code following #else, if any, is compiled.

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 #ifdef 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 #ifdef to exclude code you don’t want for a particular version of your program, the code is never compiled into byte code.