object_getInstanceVariable works for float, int, bool, but not for double?

Question

I\'ve got object_getInstanceVariable to work as here however it seems to only work for floats, bools and ints not doubles. I do suspect I\'m doing something wro

Answer 1

object_getInstanceVariable is a confused little function. It is documented that the last parameter is a void ** parameter—that is, you pass the address of a void * variable and get a pointer to the instance variable—but it is implemented as if it was a void * parameter—that is, you pass the address of the variable that you want to hold a copy of the instance variable. The problem is that the implementation ignores the size of the instance variable and just does a pointer copy. So anything that's the same size as a pointer will work perfectly. If you're running on a 32-bit architecture, only the high 32 bits will be copied. (You should witness the same behavior with a long long instance variable as well.)

The solution is to use the primary API, key-value coding, using -valueForKey:.

The other solution: If you wanted to write a fixed version, say as a category for NSObject, it would look something like this:

@implementation NSObject (InstanceVariableForKey)

- (void *)instanceVariableForKey:(NSString *)aKey {
    if (aKey) {
        Ivar ivar = object_getInstanceVariable(self, [aKey UTF8String], NULL);
        if (ivar) {
            return (void *)((char *)self + ivar_getOffset(ivar));
        }
    }
    return NULL;
}

@end

Then your code would look like this:

double myDoubleValue = *(double *)[self instanceVariableForKey:@"someDouble"];