Creating objects on the stack in Objective-C

gistfile1.m

#import <Foundation/Foundation.h>
#include <stdlib.h>
#include <objc/runtime.h>

@interface A : NSObject
@property (assign) int meaning;
@end

@implementation A

- (id)init {
	self = [super init];
	if (self) _meaning = 42;
	return self;
}

@end

#define Using(obj,cls) id obj = nil;\
  { size_t size = class_getInstanceSize([cls class]);\
  obj = alloca(size);\
  memset(obj, 0, size); }\
  object_setClass(obj, [cls class]);

int main(int argc, char *argv[]) {
	@autoreleasepool {
		Using(a, A);
		[a init];
		NSLog(@"meaning: %d", [a meaning]);
	}
}

(c.f. https://gist.github.com/iamleeg/5540103)

Unlike its C++ cousin, this doesn't cause superclass fragility, and can't suffer from the C++ member variable issue.

It is still vulnerable to the problem of floating references (since ObjC objects are expected to be reference counted, something could call -retain and then expect the object to be available later), and as with the C++ version, -dealloc won't be called (this is both a good and a bad thing, because it means some objects won't get to clean up, but it may well prevent a crash when the NSObject implementation of -dealloc tries to release the memory).