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

Question

Hello,

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

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

So that when the preprocessor parses file person.h, MY_MACRO(blahblah) expands to

#include "person.h.inline.inl"

any hints on how to do this ?

Answer 1

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 2

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.

Answer 3

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 4

Was trying to do something like this recently and the C++ compiler poked me on the back of the head and said: "Are you crazy? A Macro that calls another Macro?".

The point here is that we need to understand that the pre-processor is just a "substitution process that runs once": no "multiple iterations" whatsoever.

So, you can't.