如何包含可能存在或不存在的头文件? [英] How do you include a header file that may or may not exist?

问题描述

假设我在foo.h中定义了BAR.但是foo.h可能不存在.在没有编译器抱怨的情况下，如何包含它?

Let's assume I define BAR in foo.h. But foo.h might not exist. How do I include it, without the compiler complaining at me?

#include "foo.h"

#ifndef BAR
#define BAR 1
#endif

int main()
{
    return BAR;
}

因此，如果在foo.h中将BAR定义为2，那么如果foo.h存在，则程序将返回2，如果foo.h不存在，则程序将返回1.

Therefore, if BAR was defined as 2 in foo.h, then the program would return 2 if foo.h exists and 1 if foo.h does not exist.

推荐答案

通常，您需要执行一些外部操作才能完成此操作-例如通过执行类似搜索路径的操作(如注释中所建议)，并提供空的 foo.h 作为后备广告，或将 #include 包裹在 #ifdef HAS_FOO_H ... #endif 并通过编译器开关设置 HAS_FOO_H (用于gcc/clang等的 -DHAS_FOO_H ).

In general, you'll need to do something external to do this - e.g. by doing something like playing around with the search path (as suggested in the comments) and providing an empty foo.h as a fallback, or wrapping the #include inside a #ifdef HAS_FOO_H...#endif and setting HAS_FOO_H by a compiler switch (-DHAS_FOO_H for gcc/clang etc.).

如果您知道使用的是特定的编译器，而可移植性不是问题，请注意，某些编译器确实支持以扩展名的形式包括可能存在或不存在的文件.例如，请参见clang的 __ has_include 功能.

If you know that you are using a particular compiler, and portability is not an issue, note that some compilers do support including a file which may or may not exist, as an extension. For example, see clang's __has_include feature.

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