How to interop between Objective-C and C++?

In the world of cross-platform, it is common for the need to create bridges of communication between different languages. While working with React Native and iOS, I stumbled on a need to interop with a C++ class. This post covers how you can communicate between Objective-C and C++ code.

Using Objective-C++

You can transform your Objective-C file (.m) to Objective-C++ (.mm). This by default enables you to call any C++ code in your Objective-C++ file.

MyObjective_c_file.mm

#include “SomePrivateCppClass.h”
@implementation MyObjective_c_file
- (instancetype) init
{
 // Add your logic here
 SomePrivateCppClass::someMethod();
}
@end

Note - Sometimes you can have issues with this approach especially when your C++ interface is importing code that causes an issue for the Objective-C compiler. To overcome this you can do the next step

Interopping with a C interface to call an Objective-C function from C++

When file separation is required due to external dependencies. You can interop by using a C interface.

C.interface.h

void callFromCpp(const char *dataPayload);

MyObjective_c_file.mm

@implementation MyObjective_c_file
…
 — (void)myMethod: (NSString *) dataPayload {
// Your logic here
}
@end
void callFromCpp(const char *dataPayload) {
 NSString *convertedString = [[NSString alloc] initWithCString: dataPayload encoding:NSUTF8StringEncoding];
 
 MyObjective_c_file myInstance = [[MyObjective_c_file alloc] init];
 [MyObjective_c_file myMethod: convertedString];
}

YourCppClass.cpp

YourCppClass::WhatEverMethod() {
 callFromCpp(stringData.c_str());
}

Passing pointers with a C interface to call a C++ method.

When using a C interface, you can also pass a pointer of your C++ class in order to call it from your Objective-C++ code (This is ideal when you can’t include your C++ file due to dependencies).

In this case, you will instantiate your Objective-C class by calling a C function from your C++ method, then pass a pointer of your C++ class to your Objective-C interface for it to consume it.

C.interface.h

void initFromCpp(long castBridgePointer);

MyObjective_c_file.mm

#include “YourCppClass.h”
@implementation MyObjective_c_file
static YourCppClass *pointerToBridge;
…
- (void)storeCastBridgePointer: (long) ptr {
 pointerToBridge = (YourCppClass *) ptr;
}
@end
void initFromCpp(long castBridgePointer)
{
 MyObjective_c_file myInstance = [[MyObjective_c_file alloc] init];
 [myInstance storeCastBridgePointer: castBridgePointer];
}

YourCppClass.cpp

YourCppClass::YourCppClass() {
  // whatever logic…
  initFromCpp((long) this);
}

Conclusion

Interop between C++ and Objective-C is not too complicated. The purpose of this post is to bring both solutions, the plain Objective-C approach and C interface approach, into a single post where anyone can reference in the future