C/C++ Programmer's Guide (G06.27+, H06.03+)

Preprocessor Directives and Macros
HP C/C++ Programmer’s Guide for NonStop Systems—429301-010
12-6
#if, #elif, #ifdef, #ifndef, #else, and #endif
identifier to test, and the new line following it terminates the #ifndef directive line.
source-text is the source text that is included if identifier is not currently
defined as a macro name.
#else newline [ source-text ]
is the else group of an if section. The new line following the #else directive
terminates the directive line. source-text specifies the source text that is
included if the condition specified in the if group is not met.
#endif newline
terminates the if section. The new line following the #endif directive terminates
the directive line.
Usage Guidelines
•
When using the if directives, remember to distinguish between macro definitions
and function definitions; the #ifdef and #ifndef directives test only macro
definitions.
•
The preprocessor selects a single source-text evaluating the constant
expression following each #if or #elif directive until it finds a true, nonzero,
constant-expression. It selects all text up to its associated #elif, #else, or
#endif.
•
If all occurrences of constant-expression are false, or if no #elif directives
appear, the preprocessor selects the source text after the #else clause. If the
#else clause is omitted and all instances of constant-expression in the #if
blocks are false, no source text is selected.
Examples
1. This example shows how the #if, #else, and #endif directives interact.
Because the identifier ANSI is defined as zero (false), the #if test fails. As a
result, the compiler processes only the source text in the else group:
#define ANSI 0
#if ANSI
printf("Function prototypes supported.\n");
#else
printf("Function prototypes not supported.\n");
#endif
2. This example shows how the #elif directive works. The #if, #elif, and #else
directives are used to make one of three choices, based on the value of XCOORD
and YCOORD. Note that XCOORD and YCOORD must be defined constants.
#define XCOORD 5
#define YCOORD 5