#IFDEF | #IFNDEF ... #ENDIF Preprocessor Directive

Example See Also

Conditionally includes a set of commands at compile time if a compile-time constant is defined.

Syntax

#IFDEF | #IFNDEF ConstantName
Commands
[#ELSE
Commands]
#ENDIF

Arguments

#IFDEF

Specifies that a set of commands is included at compile time when the ConstantName is defined.

The following describe how a set of commands is included at compile time when you include #IFDEF:

If ConstantName is defined, the set of commands following #IFDEF and preceding #ELSE or #ENDIF (whichever occurs first) is included at compile time.
If ConstantName is not defined and #ELSE is included, the set of commands following #ELSE and preceding #ENDIF is included at compile time.
If ConstantName is not defined and #ELSE is not included, no commands within the #IFDEF ... #ENDIF structure are included at compile time.

#IFNDEF

Specifies that a set of commands is included at compile time when the ConstantName is not defined.

The following describe how a set of commands is included at compile time when you include #IFNDEF:

If ConstantName is not defined, the set of commands following #IFNDEF and preceding #ELSE or #ENDIF (whichever occurs first) is included at compile time.
If ConstantName is defined and #ELSE is included, the set of commands following #ELSE and preceding #ENDIF is included at compile time.
If ConstantName is defined and #ELSE is not included, no commands within the #IFNDEF ... #ENDIF structure are included at compile time.

ConstantName

Specifies the compile-time constant whose existence determines whether a set of commands is included at compile time. Compile-time constants are defined with #DEFINE.

Commands

Specifies the set of commands that is included at compile time.

Remarks

You can nest an #IFDEF | #IFNDEF ... #ENDIF structure within another #IFDEF | #IFNDEF ... #ENDIF structure.

Comments can be placed on the same line after #IFDEF, #IFNDEF, #ELSE, and #ENDIF. These comments are ignored during compilation and program execution.