How to Call an Objective-C Method from a C Member Function?

Calling an Objective-C Method from a C Member Function

Problem

An EAGLView class requires calling a member function from a C class without issues. However, within the C class, the need arises to call an Objective-C function, "[context renderbufferStorage:GL_RENDERBUFFER fromDrawable:(CAEAGLLayer*)self.layer];", which cannot be achieved using pure C syntax.

Solution

To mix Objective-C and C , proceed with caution. Here's a step-by-step approach for wrapping the Objective-C call using a C wrapper function:

1. Create a C Interface Header:

   • Create a header file named "MyObject-C-Interface.h" that defines a C wrapper function.

#include <stdio.h> // for printf
#include <stdint.h> // for uintptr_t

typedef uintptr_t Id;

// Assume a simplified EAGLView class
extern void EAGLViewDoSomethingWith(Id* poself, void *aparam);

int MyObjectDoSomethingWith(void *myObjectInstance, void *parameter) {
    printf("C wrapper function called!\n");
    // Assuming Objective-C method takes a single int argument
    return EAGLViewDoSomethingWith(myObjectInstance, 21);
}

2. Define the Objective-C Class:

   • Create a header file "MyObject.h" and implementation file "MyObject.mm".

// MyObject.h
@interface MyObject : NSObject
- (int)doSomethingWith:(void *)aParameter;
@end

// MyObject.mm
#import "MyObject.h"
@implementation MyObject
- (int)doSomethingWith:(void *)aParameter {
    // Implement Objective-C function
    return 42;
}
@end

3. Implement the C Class:

   • Include the C interface header in the C class header "MyCPPClass.h" and use the C wrapper function.

#include "MyObject-C-Interface.h"

class MyCPPClass {
public:
    void someMethod(void *objectiveCObject) {
        int result = MyObjectDoSomethingWith(objectiveCObject, nullptr);
    }
};

PIMPL Idiom for Object-Oriented Implementation

The PIMPL (Pointer to Implementation) idiom can be utilized for an object-oriented implementation:

1. Define a C Interface:

   • Create a header file "MyObject-C-Interface.h" that defines a class with wrapper methods.

#include <stdio.h> // for printf
#include <stdint.h> // for uintptr_t

typedef uintptr_t Id;

class MyClassImpl {
public:
    MyClassImpl() : self(nullptr) {}
    ~MyClassImpl() { if (self) dealloc(); }
    int doSomethingWith(void *parameter) { return 42; }
private:
    Id self;
    void dealloc() { if (self) free(self); }
};

int EAGLViewDoSomethingWith(Id* poself, void* aparam);

int MyObjectDoSomethingWith(void *myObjectInstance, void *parameter) {
    printf("C wrapper function called!\n");
    return EAGLViewDoSomethingWith(myObjectInstance, 21);
}

2. Create an Objective-C Class Interface:

   • Define a header file "MyObject.h".

@interface MyObject : NSObject
- (int)doSomethingWith:(void *)aParameter;
@end

3. Create an Objective-C Class Implementation:

   • Define an implementation file "MyObject.mm" that instantiates a MyObject instance within MyObject.

#import "MyObject.h"
@implementation MyObject {
    MyClassImpl* _impl;
}
- (int)doSomethingWith:(void *)aParameter {
    if (!_impl) _impl = [MyClassImpl new];
    return [_impl doSomethingWith:aParameter];
}
@end

4. Implement the C Class:

   • Include the C interface header in the C class header "MyCPPClass.h" and use the C wrapper function.

#include "MyObject-C-Interface.h"

class MyCPPClass {
public:
    void someMethod(void *objectiveCObject) {
        int result = MyObjectDoSomethingWith(objectiveCObject, nullptr);
    }
};

This approach provides a more isolated and flexible solution, enabling the Objective-C implementation and the C wrapper to be modified without affecting the C code.

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