NSMutableDictionary

NSMutableDictionary is a mutable collection class in the Foundation framework of Apple's Cocoa and Cocoa Touch platforms. It is the mutable counterpart to NSDictionary, enabling dynamic storage of key-value pairs where keys and values are objects. Keys must conform to NSCopying, since the dictionary copies keys; lookups are performed using isEqual: and hash.

Creation and basic usage: An NSMutableDictionary can be created with dictionary, dictionaryWithCapacity:, or initWithCapacity:. Objects are added or updated with setObject:forKey:, and values retrieved with objectForKey:. Other common operations include removeObjectForKey:, removeAllObjects, and addEntriesFromDictionary:. It inherits from NSDictionary and supports fast enumeration, allKeys, allValues, count, and copying.

Mutability and memory management: In Automatic Reference Counting (ARC), the dictionary holds strong references to keys and values. In manual memory management (MRC), you must balance retains and releases. Calling mutableCopy returns a mutable dictionary. When a key is updated, the old value is released if no other references exist.

Swift interoperability: NSMutableDictionary is available in Swift via bridging to the native Dictionary type. In many cases, native Swift dictionaries are preferred, but NSMutableDictionary is still used when interfacing with Objective-C APIs or when subclassing is required.

Thread safety and performance: NSMutableDictionary is not thread-safe for concurrent writes. Synchronization or serial queues are recommended for multi-threaded access. Using dictionaryWithCapacity: can improve performance when the approximate size is known, as internal hashing structures can be pre-sized.