Question

I have a bit of pure C++ code which is reading from Objective-C data structures with the help of a function pointer to a method in an Objective C class. I’m treating the Objective-C class instance to read from as an opaque pointer. For example, the C++ method that does the reading has a signature like this:

typedef void(*DataGetterFunc)(void * dataSource, int key, int * outValue);
...
void readData(void * dataSource, DataGetterFunc dataReadingFunc);

When I call the C++ method from Objective-C, I do the following:

MYDataStructure * objectiveCData;
cppObject->readData((__bridge void*)objectiveCData, DataGetterFuncImpl);

Finally, DataGetterFuncImpl dereferences the Objective-C class like so:

void DataGetterFuncImpl(void * dataSource, int key, int * outValue)
{
    MYDataStructure * objCData = (__bridge MYDataStructure*)dataSource;
    ...
}

Originally in DataGetterFuncImpl I was using __bridge_transfer, but then I was getting EXC_BAD_ACCESS the next time ARC called retain on MYDataStructure, So I assumed it was being over-released by the use of __bridge_transfer and changed it to just __bridge.

Are there any memory leaks I should look for by just using __bridge, or do I need to use some combination of __bridge_retain and __bridge_transfer in this case?

Answer 1

When you’re using __bridge to convert to or from objc, owership is just not affected. That means, that while you’re using the object in C++ you must make sure that there’s still a strong reference around.

If you, on the other hand, use __bridge_retain to convert to void* and __bridge_transfer to convert back to id (or any other retainable object type), you must make sure that each __bridge_retain is matched by exactly one __bridge_transfer later.