How to call Objective-C "self" from C method [duplicate] - ios

Question

This question already has answers here:
Pass instance method as function pointer to C Library
(1 answer)
How do I give C function a pointer to 'self' (calling obj) in objective-c?
(2 answers)
Closed 9 years ago.
I have Objective-C class in my iOS project that implements Objective-C and C code in the same class. I changed the extension to .mm and this part goes well. Now I want to set a C method that will call the Objective-C method in the same class. The problem I get is when I am trying to call self from the C method.
Here is the code :
void SetNotificationListeners(){
[self fireSpeechRecognition];
}
the error is :
Use of undeclared identifier 'self'
how can I manage this?

Answer 1

You have to pass the instance pointer to the C function:
void SetNotificationListeners(void *uData)
{
MyClass *obj = (__bridge MyClass *)(uData);
[obj fireSpeechRecognition];
}
- (void)myMethod
{
// Call C function from Objective-C method:
myFunction((__bridge void *)(self));
}
(The "brigde" stuff is needed only if you compile with ARC.)

Answer 2

Either give self as an argument to the function call:
void SetNotificationListeners(void *myObj){
[(MyClass*)myObj fireSpeechRecognition];
}
//in objC
SetNotificationListeners(self);
or have a global variable that holds reference
//global variable
static MyClass *myObj;
//in obj c
myObj = self;
SetNotificationListeners(); //use myObj instead of self in function
The first is better in my opinion.

Answer 3

You don’t have to change the file extension. Objective-C is a superset of C, which means you can use plain C in your Objective-C files as you please.
When you write an implementation of an Objective-C method, that method always executes in context of some particular instance, that’s the self part. You get the self in an Objective-C method automagically, but behind the scenes it’s just passed as an argument to the method, see obc_msgsend.
Your plain C function is not a part of the class (plain C functions never are), therefore there’s no instance associated with it when you call it, there’s no implicit self. If you want the function to call some instance, you have to pass the pointer to that instance explicitly. For example some of the plain C APIs have a “context” pointer you can pass when registering for a callback. See the neighboring answers for examples of this.

Answer 4

In Objective-C methods there are two parameters that are implicitly passed into each method, which are self and _cmd. This is why you can access self from within a method.
You could just pass self as an argument to your c function when calling from an Objective-C method, but with that trivial example I'm not sure why you wouldn't just use a method.
You do not need to change the file extension to .mm unless you are using Objective-C++