Is there a way to conditionally include a header or import a module depending on a switch/flag?

Question

I've tried:

#ifdef USE_CPP20_MODULES
#define IMPORT_OR_INCLUDE(module_name, include_filepath) \
import module_name;
#else
#define IMPORT_OR_INCLUDE(module_name, include_filepath) \
#include include_filepath
#endif

But none of the compilers compiler it:

Clang:

    Output of x86-64 clang 19.1.0 (Compiler #1)

<source>:6:2: error: '#' is not followed by a macro parameter
    6 | #include include_filepath
      |  ^

GCC:

<source>:5:56: error: '#' is not followed by a macro parameter
    5 | #define IMPORT_OR_INCLUDE(module_name, include_filepath) \

I do not understand why the preprocessor fails.
I'm guessing you can't use #include as the definition/replacement of a macro?

Answer 1

# has special meaning for the preprocessor. It's the stringize operator.

[cpp.stringize]:

15.6.3 The # operator

1. Each # preprocessing token in the replacement list for a function-like macro shall be followed by a parameter as the next preprocessing token in the replacement list.

Since include is not a parameter in the function-like macro, substitution fails.

You can't add a parameter taking #include as an argument either:

[cpp.replace]:

If there are sequences of preprocessing tokens within the list of arguments that would otherwise act as preprocessing directives, the behavior is undefined.

---

You could put all imports for this file in one header and all the includes in a different header and then #include the the one you want according to USE_CPP20_MODULES:

#ifdef USE_CPP20_MODULES
#define HEADER_NAME "tu_imports.h"
#else
#define HEADER_NAME "tu_includes.h"
#endif
#include HEADER_NAME

Or just:

#ifdef USE_CPP20_MODULES
#include "tu_imports.h"
#else
#include "tu_includes.h"
#endif

Or simply do it directly where you need it:

#ifdef USE_CPP20_MODULES
import module_a;
import module_b;
#else
#include "a.h"
#include "b.h"
#endif