Styjun

I have a C++ class MyClass that declare a public enum type MyEnum, and I want to use that enum in a C file. How can I do that ?

I tried to declare my functions in a C++ file and then put everything as extern "C", but sadly I am using some functions defined in big_hugly_include.h and this header does not like being included as external "C" (it gives me a template with C linkage error).

I cannot (don't want to) change this include, and I need it because it defines my_function_from_big_include. Am I stuck ?

my_class_definition.h :

class MyClass

public:
 // I would like to keep it that way as it is mainly used in C++ files
 typedef enum
 
 MY_ENUM_0,
 MY_ENUM_1,
 MY_ENUM_2
 MyEnum;
;

Try 1 : my_c_function_definition.c :

#include "my_class_definition.h"

// I cannot remove this header
#include "big_hugly_include.h"

// foo is called in other C files
void foo()

 // I need to call this function with the enum from the C++ class
 // This doesn't work (class name scope does not exist in C)
 my_function_from_big_include(MyClass::MyEnum::MY_ENUM_0);

Try 2 : my_c_function_definition.cpp :

#include "my_class_definition.h"

extern "C"


// Error template with C linkage
#include "big_hugly_include.h"

// foo is called in other C files
void foo()

 // That would be ideal
 my_function_from_big_include(MyClass::MyEnum::MY_ENUM_0);


// end of extern "C"

Edited in response to @artcorpse

Try 3 : my_c_function_definition.cpp :

#include "my_class_definition.h"

// Error multiple definition of [...]
// Error undefined reference to [...]
#include "big_hugly_include.h"

extern "C"

// foo is called in other C files
void foo()

 // That would be ideal
 my_function_from_big_include(MyClass::MyEnum::MY_ENUM_0);


// end of extern "C"

I want to use that enum in a C file. How can I do that?

Enum concept in C++ originates from C, so only thing you have to do is to isolate definition this enum from pure cpp API which is not known to C (remember about name mangling, see below).

Since C do not know in class/struct enums you can't use them. You have to define global scope enumeration or create such enumeration which will map C++ specific enumeration.

So create separate header file where shared API should be located. Do something like this:

// shared C, C++ header
#ifdef __cplusplus
extern "C" 

#endif

enum YourMagicEnum 
 YourMagicEnumAValue,
 YourMagicEnumBValue,
 YourMagicEnumCValue,
;

void someFunction(YourMagicEnum x);

#ifdef __cplusplus
 // extern "C"
#endif

Now this extern "C" is needed only for functions to disable name mangling (in C++ you can do function overloads, so compiler generates names which contain arguments type information).

When defining such function it should also have the extern "C" in front of that definition.

And remember in that header only C specific features and functions can be placed.

Also remember that VLA (Variable-length array) is in C standard, but not in C++ standard (most compiler support VLA for C++).

For more information see this page.

Your Try2 is pretty close to a solution. Try to move the include outside extern "C".
I usually just mark each function individually:

extern "C" void foo() 

...

This has the benefit of just exporting the one symbol as a C symbol, instead of trying to convert everything.