Using #define to include another file in C++/C

Question

I want to define a macro which includes another header file like so:

#define MY_MACRO (text) #include \"__FILE__##_inline.inl\"

So that whe

Answer 1

#if 0 /*Windows*/
#define MKDIR_ENABLER <direct.h>
#define MY_MKDIR(x,y) _mkdir((x))
#else /*Linux*/
#define MKDIR_ENABLER <sys/stat.h>
#define MY_MKDIR(x,y) mkdir((x),(y))
#endif

#include MKDIR_ENABLER

int main(void)
{
    MY_MKDIR("more_bla",0644);
    return 0;
}

This code includes the appropriate header file for mkdir (because it's different on UNIX and Windows) and introduces a nice wrapper for it.

Answer 2

You cannot use __FILE__ because that is already quoted, and #include doesn't support string concatenation. But you can use macros after #include:

#define STRINGIZE_AUX(a) #a
#define STRINGIZE(a) STRINGIZE_AUX(a)
#define CAT_AUX(a, b) a##b
#define CAT(a, b) CAT_AUX(a, b)
#define MY_MACRO(file, name) STRINGIZE(CAT(file, CAT(name, _inline.inl)))
#include MY_MACRO(aaaa, qqq)

You should use the equivalent Boost.Preprocessor macros instead of CAT and STRINGIZE to prevent global namespace pollution.

Answer 3

It's not possible to use #define to construct other preprocessor directives, unless you run the preprocessor twice.

But in your case even running the preprocessor twice won't help because the #include must be a single string of the form "..." or <...>.

Answer 4

You can't write other pre-processor directives using the pre-processor. However, I believe you could define just the file name:

#define MY_MACRO(name) "__FILE__##name_inline.inl"

#include MY_MACRO(name)

The pre-processor runs multiple times until there are no further substitutions it can make, so it should expand the name first and then #include the referenced file.

EDIT: I just tried it and the pre-processor can't handle the quotes like that.

#define MY_MACRO(x) <__FILE__##x_inline.inl>
#include MY_MACRO(foo)

works OK, but <> may not be what you wanted.

EDIT2: As pointed out by sth in comments, the __FILE__ does not expand correctly, which makes this probably not what you want after all. Sorry.