HP C/aC++ for PA-RISC Software manual Conditional Compilation #if, #ifdef, .. #endif

HP aC++ predefines the following predicates:

•#assert system(unix)

•#assert model(lp64) // when +DA2.0W is used

•#assert model(ilp32) // default

•#assert endian(big)

Example:

int main()

{

#assert dimensions(three) // Set predicate and token to true. #if #dimensions(two)

#error "May not compile in 2 dimensions" #endif

#if #dimensions(three) int x, y, z;

#endif

#unassert dimensions	// Set predicate and all tokens to false.
}

Conditional Compilation (#if, #ifdef, .. #endif)

Conditional compilation directives allow you to delimit portions of code that are compiled only if a condition is true.

Syntax

conditional-directive ::=

#if	constant-expression newline
#ifdef	identifier newline [group]
#ifndef	identifier newline [group]
#else	newline [group]
#elif	constant-expression newline [group]
#endif

Here, constant-expressionmay also contain the defined operator:

defined identifier

defined (identifier)

The constant-expressionis like other C++ integral constant expressions except that all arithmetic is carried out in long int precision. Also, the expressions cannot use the sizeof operator, a cast, an enumeration constant, or a const object.

Description

You can use #if, #ifdef, or #ifndef to mark the beginning of the block of code that will only be compiled conditionally. An #else directive optionally sets aside an alternative group of statements. You mark the end of the block using an #endif directive.

The following #if directive illustrates the structure of conditional compilation:

#if constant-expression

...

(Code that compiles if the expression evaluates to a nonzero value.)

...

#else

...

(Code that compiles if the expression evaluates to zero.)

126 Preprocessing Directives