Enable preprocessor flags during build

Development C++ interoperability
1

Hey folks C/C++ interop question:
I have a C++ header file that only exposes a class if a preprocessor flag (I think that’s how you call it?) is set:

#ifdef UNIX_ENABLED

class Some_Class {
...
}
#endif // UNIX_ENABLED

When I call the swift compiler:
swiftc MyApp.swift -cxx-interoperability-mode=default -Xcc -std=c++17 -I cxx -c -parse-as-library
and try to use Some_Class in MyApp.swift it obviously can’t find that class. I tried using -D UNIX_ENABLED but that didn’t help

Any ideas how I could get this compiling?

Note: When I don’t use Some_Class in MyApp.swift the project builds perfectly fine

This is part of a bigger project which uses SCons (:melting_face:). In pure C++ areas of the project other developers used the following SCons code to enable it:
env.Append(CPPDEFINES=["UNIX_ENABLED"])

Many thanks in advance!

2

Could you try passing -Xcc -D -Xcc UNIX_ENABLED to swiftc to make sure it forwards the -D to clang?

1 Like
3

Ohh that did the trick! Thank you very much!

4

Small follow-up: Is it possible to view the "generated" Swift code? It would really help me seeing how Swift interprets the C++ code. In Xcode you can cmd-click the imported module to view the Swift representation of the C++ code - is this possible outside of Xcode?

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