swift-style as in ObjC

as.m

#import <Foundation/Foundation.h>

#define as_checked(EXPR, KLASS) ({ id _obj = EXPR; NSCAssert([_obj isKindOfClass:[KLASS class]], @"Cannot cast %@ to %@", NSStringFromClass([_obj class]), NSStringFromClass([KLASS class])); _obj; })

#define as_option(EXPR, KLASS) ({ id _obj = EXPR; if (![_obj isKindOfClass:[KLASS class]]) _obj = nil; _obj; })


@interface NSObject(As)

- (instancetype)asChecked:(Class)klass;
- (instancetype)asOption:(Class)klass;

@end

@implementation NSObject(As)

- (instancetype)asOption:(Class)klass {
	return [self isKindOfClass:klass] ? self : nil;
}

- (instancetype)asChecked:(Class)klass {
	NSAssert([self isKindOfClass:klass], @"Cannot cast %@ to %@", NSStringFromClass([self class]), NSStringFromClass(klass));
	return self;
}

@end


int main(int argc, char *argv[]) {
	@autoreleasepool {
		NSString *x = @"x";
		NSNumber *y = @(1);
		
		NSString *xx = as_option(x, NSString); // xx == x
		NSString *yy = as_option(y, NSString); // yy == nil

		NSLog(@"%@ ; %@", xx, yy);
		
		xx = [x asOption:NSString.class]; // xx = x
		yy = [y asOption:NSString.class]; // yy = nil
		
		NSLog(@"%@ ; %@", xx, yy);
		
		xx = as_checked(x, NSString.class); // xx == x
		xx = [x asChecked:NSString.class];
		
		yy = as_checked(y, NSString); // Cannot cast __NSCFNumber to NSString
		yy = [y asChecked:NSString.class]; // Cannot cast __NSCFNumber to NSString
	}
}