The conditional compilation directives are used to conditionally include or exclude portions of a source file. pp-conditional :: pp-if-sectionpp-elif-sectionspp-else-sectionpp-endifpp-if-section :: whitespace#whitespaceifwhitespacepp-expressionpp-new-lineconditional-sectionpp-elif-sections :: pp-elif-sectionpp-elif-sectionspp-elif-sectionpp-elif-section :: whitespace#whitespaceelifwhitespacepp-expressionpp-new-lineconditional-sectionpp-else-section :: whitespace#whitespaceelsepp-new-lineconditional-sectionpp-endif :: whitespace#whitespaceendifpp-new-lineconditional-section :: input-sectionskipped-sectionskipped-section :: skipped-section-partskipped-sectionskipped-section-partskipped-section-part :: skipped-charactersnew-linepp-directiveskipped-characters :: whitespacenot-number-signinput-charactersnot-number-sign :: Any input-character except # [Note: 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. end note] 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. [Example: The following example illustrates how conditional compilation directives can nest: Except for pre-processing directives, skipped source code is not subject to lexical analysis. For example, the following is valid despite the unterminated comment in the #else section: Note, however, that pre-processing directives are required to be lexically correct even in skipped sections of source code. Pre-processing directives are not processed when they appear inside multi-line input elements. For example, the program: results in the output: In peculiar cases, the set of pre-processing directives that is processed might depend on the evaluation of the pp-expression. The example: always produces the same token stream (class Q { }), regardless of whether or not X is defined. If X is defined, the only processed directives are #if and #endif, due to the multi-line comment. If X is undefined, then three directives (#if, #else, #endif) are part of the directive set. end example]