What is a dictionary?Bridging Between Dictionary and NSDictionary

A dictionary is a type of hash table, providing fast access to the entries it contains. Each entry in the table is identified using its key, which is a hashable type such as a string or number. You use that key to retrieve the corresponding value, which can be any object. In other languages, similar data types are known as hashes or associated arrays.

Create a new dictionary by using a dictionary literal. A dictionary literal is a comma-separated list of key-value pairs, in which a colon separates each key from its associated value, surrounded by square brackets. You can assign a dictionary literal to a variable or constant or pass it to a function that expects a dictionary.

Here’s how you would create a dictionary of HTTP response codes and their related messages:

var responseMessages = [200: "OK",
                        403: "Access forbidden",
                        404: "File not found",
                        500: "Internal server error"]

The responseMessages variable is inferred to have type [Int: String]. The Key type of the dictionary is Int, and the Value type of the dictionary is String.

To create a dictionary with no key-value pairs, use an empty dictionary literal ([:]).

var emptyDict: [String: String] = [:]

Any type that conforms to the Hashable protocol can be used as a dictionary’s Key type, including all of Swift’s basic types. You can use your own custom types as dictionary keys by making them conform to the Hashable protocol.


You can bridge between Dictionary and NSDictionary using the as operator. For bridging to be possible, the Key and Value types of a dictionary must be classes, @objc protocols, or types that bridge to Foundation types.

Bridging from Dictionary to NSDictionary always takes O(1) time and space. When the dictionary’s Key and Value types are neither classes nor @objc protocols, any required bridging of elements occurs at the first access of each element. For this reason, the first operation that uses the contents of the dictionary may take O(n).

Bridging from NSDictionary to Dictionary first calls the copy(with:) method (- copyWithZone: in Objective-C) on the dictionary to get an immutable copy and then performs additional Swift bookkeeping work that takes O(1) time. For instances of NSDictionary that are already immutable, copy(with:) usually returns the same dictionary in O(1) time; otherwise, the copying performance is unspecified. The instances of NSDictionary and Dictionary share buffer using the same copy-on-write optimization that is used when two instances of Dictionary share buffer.


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