setObjectforKey

setObject:forKey: is an instance method of NSMutableDictionary in the Foundation framework. It adds or updates a key-value pair in the dictionary, using the provided key to identify the entry and the provided object as the value. If the key already exists, the existing value is replaced with the new one.

Signature and constraints: - (void)setObject:(id)anObject forKey:(id<NSCopying>)aKey;. The key must conform to NSCopying, and neither anObject nor aKey may be nil. Passing nil for either argument will raise an exception. The dictionary retains a strong reference to the object.

Usage examples: Objective-C code often uses setObject:forKey: with a mutable dictionary. For example, NSMutableDictionary *dict = [NSMutableDictionary dictionary]; [dict setObject:@(1) forKey:@\"count\"]; id value = [dict objectForKey:@\"count\"]; Modern Objective-C also allows subscripting as a shorthand, such as dict[@\"count\"] = @1, which is functionally equivalent.

Context and alternatives: This method is specific to mutable dictionaries; NSDictionary, being immutable, does not support setObject:forKey:. In Swift, dictionaries generally use native subscript syntax (dict[key] = value) for mutable dictionaries, though bridging with NSMutableDictionary and setObject:forKey: remains available when interoperating with Objective-C APIs. The method plays a foundational role in managing dynamic collections of objects in Cocoa and Cocoa Touch applications.