Question

I’m making a utility to get/set property values when the property has custom-named getters and setters. You can see the full context at line 279 here. The relevant snippet is here:

- (id) getFrom:(id) object {
    NSMethodSignature *methodSig = [[object class] instanceMethodSignatureForSelector:[self getter]];
    NSInvocation *inv = [NSInvocation invocationWithMethodSignature:methodSig];
    [inv setSelector:[self getter]];
    [inv setTarget:object];
    [inv invoke];

    if ([self isObject]) {
        id returnValue;
        [inv getReturnValue:&returnValue];
        return returnValue;
    } else {
        void *buffer;
        NSUInteger length = [methodSig methodReturnLength];
        buffer = (void *)malloc(length);
        [inv getReturnValue:buffer];
        NSValue *value = [NSValue valueWithBytes:buffer objCType:[methodSig methodReturnType]];
        //FIXME: Memory leak for buffer!  But if we free it, [value getValue:] is a dangling pointer.
        //free(buffer)
        return value;
    }
}    

The problem is that when the property is a scalar, I want to return an NSValue (much like Key-Value coding). However, the return value for NSInvocation is returned by reference, and according to the apple documentation (see the bottom), I can’t free the memory associated with the scalar while the NSValue still exists — but I’m returning the NSValue, so I don’t know when to free the memory.

Am I reading the documentation wrong? Does NSValue handle this automatically somehow? Or how do I free the memory properly in this situation?

Answer 1

Your must free buffer. +valueWithBytes:objCType: copies the buffer sent in, so you can free buffer when you are done.

You could also optionally use a NSData object here, like this:

NSData *data = [[NSData alloc] initWithBytesNoCopy:buffer length:length freeWhenDone:YES]; 

This will mean that when the NSData object is deallocated, so will the buffer you created.

Proof that +valueWithBytes:objCType: copies the buffer:

int *buffer = malloc(sizeof(int));
buffer[0] = 50;

NSValue *value = [NSValue valueWithBytes:buffer objCType:@encode(int)];

// sribble 'buffer'
buffer[0] = 1000;
free(buffer);

int test = 1000;
[value getValue:&test];

printf("%i", test);  // outputs 50