Conditional inclusion
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支持有条件的预处理器的编译源文件的部分。这种行为是控制
#if，#else，#elif，#ifdef，#ifndef和#endif指令.原文：
The preprocessor supports conditional compilation of parts of source file. This behavior is controlled by
#if, #else, #elif, #ifdef, #ifndef and #endif directives.目录 |
[编辑] 语法
#if expression
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#elif expression
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#ifdef expression
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#ifndef expression
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#else expression
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#endif expression
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[编辑] 解释
的条件的预处理块开始
#if，#ifdef或#ifndef指令，然后任选地包括任意数量的#elif指令，然后任选地包括至多1#else指令和终止与#endif指令。内的任何条件预处理块分别进行处理.原文：
The conditional preprocessing block starts with
#if, #ifdef or #ifndef directive, then optionally includes any number of #elif directives, then optionally includes at most one #else directive and is terminated with #endif directive. Any inner conditional preprocessing blocks are processed separately.#if，#elif，#else，#ifdef#ifndef指令控制的代码块，直到第一个#elif，#else，#endif指令不属于任何内部条件预处理块. 原文：
Each of
#if, #elif, #else, #ifdef and #ifndef directives control code block until first #elif, #else, #endif directive not belonging to any inner conditional preprocessing blocks. #if，#ifdef#ifndef指令测试指定条件（见下文），如果它的值为true，编译控制的代码块。在这种情况下，后续的#else和#elif指令将被忽略。否则，如果指定的条件判断为假，则控制的代码块被跳过，随后的#else或#elif指令（如果有的话）处理。在前者的情况下，由#else指令控制的代码块被无条件地编译。在后者的情况下，#elif指令的作用，如果它是#if指令：检查条件，编译或跳过控制基于该结果的代码块，并且在后者的情况下处理后续#elif和#else指令。 #endif指令终止的条件预处理块.原文：
#if, #ifdef and #ifndef directives test the specified condition (see below) and if it evaluates to true, compiles the controlled code block. In that case subsequent #else and #elif directives are ignored. Otherwise, if the specified condition evaluates false, the controlled code block is skipped and the subsequent #else or #elif directive (if any) is processed. In the former case, the code block controlled by the #else directive is unconditionally compiled. In the latter case, the #elif directive acts as if it was #if directive: checks for condition, compiles or skips the controlled code block based on the result, and in the latter case processes subsequent #elif and #else directives. The conditional preprocessing block is terminated by #endif directive.[编辑] 状况评估
[编辑] #if, #elif
expression是一个常量表达式，只用文字定义的标识符，使用
#define指令。任何标识，这是没有文字的，不使用 #define指令定义，计算结果为“0”. 表达式可以包含一元运算符的形式
definedidentifier或defined (identifier)1定义使用identifier指令和“0”，否则返回 #define。如果expression计算结果为非零值，包括控制代码块，否则跳过。如果有任何使用的标识符是不是一个常数，它会被替换0.原文：
The expression may contain unary operators in form
defined identifier or defined (identifier) which return 1 if the identifier was defined using #define directive and 0 otherwise. If the expression evaluates to nonzero value, the controlled code block is included and skipped otherwise. If any used identifier is not a constant, it is replaced with 0.[编辑] #ifdef, #ifndef
#ifdefidentifier基本上等同于#if defined(identifier).原文：
#ifdef identifier is essentially equivalent to #if defined( identifier).#ifndefidentifier基本上等同于#if !defined(identifier).原文：
#ifndef identifier is essentially equivalent to #if !defined( identifier).[编辑] 示例
#define ABCD 2 #include <iostream> int main() { #ifdef ABCD std::cout << "1: yes\n"; #else std::cout << "1: no\n"; #endif #ifndef ABCD std::cout << "2: no1\n"; #elif ABCD == 2 std::cout << "2: yes\n"; #else std::cout << "2: no2\n"; #endif #if !defined(DCBA) && (ABCD < 2*4-3) std::cout << "3: yes\n"; #endif }
输出：
1: yes 2: yes 3: yes
