Mixing C functions in an Objective-C class

Question

I am writing an Objective-C class but it uses an API written in C. This is mostly fine as mixing C calls with Objective-C calls causes few problems.

However one of the API call requires a call back method (example):

success = CFHostSetClient(host, MyCFHostClientCallBack, &context); 

Where MyCFHostClientCallBack is a C function defined like this:

static void MyCFHostClientCallBack(CFHostRef host, CFHostInfoType typeInfo, const CFStreamError *error, void *info); 

1. Can/How do I call an Objective-C method in place of this?
2. Can/Should I mix C functions with my Objective-C calls?
3. How do I mix C functions with Objective-C methods?

Answer 1

Mixing C and Objective-C methods and function is possible, here is a simple example that uses the SQLite API within an iPhone App: (course site)

Download the Zip file (09_MySQLiteTableView.zip)

C functions need to be declared outside of the @implementation in an Objective-C (.m) file.

int MyCFunction(int num, void *data) {      //code here... }  @implementation  - (void)MyObjectiveCMethod:(int)number withData:(NSData *)data {       //code here }  @end 

Because the C function is outside of the @implementation it cannot call methods like

[self doSomething] 

and has no access to ivars.

This can be worked around as long as the call-back function takes a userInfo or context type parameter, normally of type void*. This can be used to send any Objective-C object to the C function.

As in the sample code, this can be manipulated with normal Objective-C operations.

In addition please read this answer: Mixing C functions in an Objective-C class

Answer 2

To call Objective-C code from a C callback I would use something like:

void * refToSelf; int cCallback() {     [refToSelf someMethod:someArg]; }  @implementation SomeClass - (id) init {      self = [super init];      refToSelf = self; } - (void) someMethod:(int) someArg { }