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
{
}

Answer 3

Can/How do I call an Objective-C method in place of this?

You cannot.

Can/Should I mix C function in with my Objective-C call?

Yes. Write a C function and use that as the callback to the CF function.

How do I mix C function with Objective-C methods?

You can set self as the info pointer in your context structure. That will be passed to the callback. Then, in the callback, cast the info pointer back to id:

MyClass *self = (id)info;

You can then send self messages. You still can't directly access instance variables, though, since a C function is outside of the @implementation section. You'll have to make them properties. You can do this with a class extension. (Contrary to what that document says, you would not declare the extension inside @implementation, but in the same file with it, generally right above it.)

Answer 4

What I've always found helpful in this situation is to make an Obj-C wrapper on top of the C API. Implement what you need to using C functions, and build an Objective-C class (or two) on top of it, so that's all the outside world will see. For example, in the case of a callback like this, you might make a C function that calls Obj-C delegate methods on other objects.