Chapter 18 Unsafe code
pp-elif-section: whitespaceopt # whitespaceopt elif whitespace pp-expression pp-new-line conditional-sectionopt
pp-else-section: whitespaceopt # whitespaceopt else pp-new-line conditional-sectionopt
pp-endif: whitespaceopt # whitespaceopt endif pp-new-line
conditional-section: input-section skipped-section
skipped-section: skipped-section-part skipped-section skipped-section-part
skipped-section-part: skipped-charactersopt new-line pp-directive
skipped-characters: whitespaceopt not-number-sign input-charactersopt
not-number-sign: Any input-character except #
As indicated by the syntax, conditional compilation directives must be written as sets consisting of, in order, an #if directive, zero or more #elif directives, zero or one #else directive, and an #endif directive. Between the directives are conditional sections of source code. Each section is controlled by the immediately preceding directive. A conditional section may itself contain nested conditional compilation directives provided these directives form complete sets.
A pp-conditional selects at most one of the contained conditional-sections for normal lexical processing:
The pp-expressions of the #if and #elif directives are evaluated in order until one yields true. If an expression yields true, the conditional-section of the corresponding directive is selected.
If all pp-expressions yield false, and if an #else directive is present, the conditional-section of the #else directive is selected.
Otherwise, no conditional-section is selected.
The selected conditional-section, if any, is processed as a normal input-section: the source code contained in the section must adhere to the lexical grammar; tokens are generated from the source code in the section; and pre-processing directives in the section have the prescribed effects.
The remaining conditional-sections, if any, are processed as skipped-sections: except for pre-processing directives, the source code in the section need not adhere to the lexical grammar; no tokens are generated from the source code in the section; and pre-processing directives in the section must be lexically correct but are not otherwise processed. Within a conditional-section that is being processed as a skipped-section, any nested conditional-sections (contained in nested #if...#endif and #region...#endregion constructs) are also processed as skipped-sections.
The following example illustrates how conditional compilation directives can nest:
#define Debug// Debugging on #undef Trace// Tracing off
Copyright Microsoft Corporation 1999-2003. All Rights Reserved.49