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    <p>Within the header file for the <code>Log</code> class you need to provide a callback mechanism with C-linkage that can be used by C++ and you need to guard against C++ seeing the Objective-C stuff. This header can then be shared by both C++ and Objective-C. An alternative to this is to provide a separate header file and implementation file solely for the use of C++, however that adds more cruft to the implementation.</p> <p>As per your comment, I have added C++ accessible methods to create and destroy the Objective-C <code>Log</code> object, however I'm certain for this to work you'll have to use MRR rather than ARC so that you can manage the lifetime. So you you will need to compile <code>Log.m</code> with <code>-fobjc-no-arc</code>.</p> <p>Log.h:</p> <pre><code>#ifdef __OBJC__ #import &lt;Foundation/Foundation.h&gt; #else // !__OBJC__ #include &lt;stdarg.h&gt; // And maybe other C++ headers... #endif // __OBJC__ typedef void *ObjcLog; #ifdef __cplusplus extern "C" { #endif extern ObjcLog *logCreate(const char *filename); extern void logDestroy(ObjcLog *logObj); extern void logInfo(ObjcLog *logObj, const char *msg, ...); extern void logError(ObjcLog *logObj, const char *msg, ...); #ifdef __cplusplus } // extern "C" #endif #ifdef __OBJC__ @interface Log : NSObject { // stuff } // Other stuff @end #endif // __OBJC__ </code></pre> <p>Log.m:</p> <pre><code>#import "Log.h" ObjcLog *logCreate(const char *filename) { // Assumes [Log initWithFilename:] Log *log = [[Log alloc] initWithFilename:[NSString stringWithUTF8String:filename]]; return (ObjcLog *)log; } void logDestroy(ObjcLog *logObj) { Log *log = (Log *)logObj; [log release]; } void logInfo(ObjcLog *logObj, const char *msg, ...) { char buffer[8192]; va_list va; va_start(va, msg); vsprintf(buffer, msg, va); va_end(va); Log *log = (Log *)logObj; // Assumes [Log info:] [log info:[NSString stringWithUTF8String:buffer]]; } void logError(ObjcLog *logObj, const char *msg, ...) { char buffer[8192]; va_list va; va_start(va, msg); vsprintf(buffer, msg, va); va_end(va); Log *log = (Log *)context; // Assumes [Log error:] [log error:[NSString stringWithUTF8String:buffer]]; } @implementation Log ... @end </code></pre> <p>Your C++ code should then be able to use the Objective-C <code>Log</code> object like this:</p> <pre><code>ObjcLog *logObj = logCreate("/path/to/file.log"); ... logInfo(logObj, "The answer is %d. What is the question?", 42); ... logDestroy(logObj); </code></pre> <p>I would probably favour the creation of a C++ wrapper class for the Objective-C class, which will make it easier to manage the lifetime of the object and simplify access to it. However I will avoid adding it here as it will likely just complicate something that is already over-complicated.</p>
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    1. COIs there any way I can init it in the .cpp file? If you could share the code for it that will be great.
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    2. CO@MartiMarkov OK, I have updated my answer. That actually makes the implementation much neater and more obvious. Take note about not using ARC when implementing `Log`.
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    3. COWhat do you mean by that? Log is mainly static (singleton if not accessing the static methods). You mean for the ARC to not remove it and the c code to use it after it was removed?
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