"Safe" casting in Objective-C using `instancetype`.

gistfile1.m

#import <Foundation/Foundation.h>

@interface NSObject (Cast)
+ (instancetype)cast:(id)object;
@end

@implementation NSObject (Cast)
+ (instancetype)cast:(id)object
{
    return [object isKindOfClass:self] ? object : nil;
}
@end

void example()
{
    id a = @"a";
    NSObject *b = [NSObject cast:a]; // okay, @"a" is an NSObject.
    NSString *c = [NSString cast:a]; // okay, @"a" is an NSString.
    NSNumber *d = [NSNumber cast:a]; // nil; a string is not a number
    NSCAssert(b && c && !d, @"objects are only nil where casts failed");
}

Nice catch;
Thanks