HP C/aC++ for PA-RISC Software manual Using the defined Operator, Using the #if Directive

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Using the defined Operator

You can use the defined operator in the #if directive to use expressions that evaluate to 0 or 1 within a preprocessor line. This saves you from using nested preprocessing directives.

The parentheses around the identifier are optional. Below is an example:

#if defined (MAX) && ! defined (MIN)

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Without using the defined operator, you would have to include the following two directives to perform the above example:

#ifdef max #ifndef min

Using the #if Directive

The #if preprocessing directive has the form:

#ifconstant-expression

Use #if to test an expression. HP aC++ evaluates the expression in the directive. If the expression evaluates to a non-zero value (TRUE), the code following the directive is included. Otherwise, the expression evaluates to FALSE and HP aC++ ignores the code up to the next #else, #endif, or #elif directive.

All macro identifiers that appear in the constant-expression are replaced by their current replacement lists before the expression is evaluated. All defined expressions are replaced with either 1 or 0 depending on their operands.

The #endif Directive

Whichever directive you use to begin the condition (#if, #ifdef, or #ifndef), you must use #endif to end the if section.

Using the #ifdef and #ifndef Directives

The following preprocessing directives test for a definition:

#ifdef identifier

#ifndef identifier

These preprocessing directives behave like the #if directive, but #ifdef is considered true if the identifier was previously defined using a #define directive or the -Doption. #ifndef is considered TRUE if the identifier is not defined yet.

Nesting Conditional Compilation Directives

Use the #else directive to specify an alternative section of code to be compiled if the #if, #ifdef, or #ifndef conditions fail. The code after the #else directive is included if the code following any of the #if directives is not included.

Using the #else Directive

Use the #else directive to specify an alternative section of code to be compiled if the #if, #ifdef, or #ifndef conditions fail. The code after the #else directive is included if the code following any of the #if directives is not included.

Using the #elif Directive

The #elif constant-expression directive tests whether a condition of the previous #if, #ifdef, or #ifndef was false. #elif has the same syntax as the #if directive and can be used in place of an #else directive to specify an alternative set of conditions.

Overview of the Preprocessor 127