C 预处理器：基于定义包含

Question

如何根据定义的字符串的值包含一个文件或另一个文件？

这不起作用：

#define VAR VALUE_A

#if VAR == "VALUE_A"
    #include "a.h"
#elif VAR == "VALUE_B"
    #include "b.h"
#endif

如果它很重要，我实际上并没有定义VAR，我只是通过 命令行将它传递下来gcc -D NAME=VALUE。

Answer 1

该==运算符不比较字符串。但是您还有其他几个选项来配置您的包含内容。除了其他答案中已经提到的解决方案之外，我喜欢这个解决方案，因为我认为它非常不言自明。

/* Constant identifying the "alpha" library. */
#define LIBRARY_ALPHA 1

/* Constant identifying the "beta" library. */
#define LIBRARY_BETA 2

/* Provide a default library if the user does not select one. */
#ifndef LIBRARY_TO_USE
#define LIBRARY_TO_USE LIBRARY_ALPHA
#endif

/* Include the selected library while handling errors properly. */
#if LIBRARY_TO_USE == LIBRARY_ALPHA
#include <alpha.h>
#elif LIBRARY_TO_USE == LIBRARY_BETA
#define BETA_USE_OPEN_MP 0  /* You can do more stuff than simply include a header if needed. */
#include <beta.h>
#else
#error "Invalid choice for LIBRARY_TO_USE (select LIBRARY_ALPHA or LIBRARY_BETA)"
#endif

您的用户现在可以使用以下命令进行编译：

/* Constant identifying the "alpha" library. */
#define LIBRARY_ALPHA 1

/* Constant identifying the "beta" library. */
#define LIBRARY_BETA 2

/* Provide a default library if the user does not select one. */
#ifndef LIBRARY_TO_USE
#define LIBRARY_TO_USE LIBRARY_ALPHA
#endif

/* Include the selected library while handling errors properly. */
#if LIBRARY_TO_USE == LIBRARY_ALPHA
#include <alpha.h>
#elif LIBRARY_TO_USE == LIBRARY_BETA
#define BETA_USE_OPEN_MP 0  /* You can do more stuff than simply include a header if needed. */
#include <beta.h>
#else
#error "Invalid choice for LIBRARY_TO_USE (select LIBRARY_ALPHA or LIBRARY_BETA)"
#endif