This creates an Objective-C object in the stack. ARC code.

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 {
    if ([super init]) {
        _meaning = 42;
    }
    return self;
}
 
@end
 
 
int main(int argc, char *argv[]) {
    @autoreleasepool {
    
        // allocate and zero stack memory
        size_t size = class_getInstanceSize([A class]);
        id obj = (__bridge_transfer id) alloca(size);
        memset((__bridge void*)obj, 0, size);
        
        // set class and initialize the object
        object_setClass(obj, [A class]);
        obj = [obj init];
        
        NSLog(@"meaning: %d", [obj meaning]);
        
        // transfer ownership from ARC to CF so ARC doesn't 
        // try to improperly free the stack allocated memory
        CFTypeRef ref = (__bridge_retained CFTypeRef) obj;
    }
}