ExpressibleByDictionaryLiteral

A dictionary literal is a simple way of writing a list of key-value pairs. You write each key-value pair with a colon (:) separating the key and the value. The dictionary literal is made up of one or more key-value pairs, separated by commas and surrounded with square brackets.

To declare a dictionary, assign a dictionary literal to a variable or constant:

let countryCodes = ["BR": "Brazil", "GH": "Ghana",
                    "JP": "Japan", "US": "United States"]
// 'countryCodes' has type [String: String]

print(countryCodes["BR"]!)
// Prints "Brazil"

When the context provides enough type information, you can use a special form of the dictionary literal, square brackets surrounding a single colon, to initialize an empty dictionary.

var frequencies: [String: Int] = [:]
print(frequencies.count)
// Prints "0"

Conforming to the ExpressibleByDictionaryLiteral Protocol

To add the capability to be initialized with a dictionary literal to your own custom types, declare an init(dictionaryLiteral:) initializer. The following example shows the dictionary literal initializer for a hypothetical CountedSet type, which uses setlike semantics while keeping track of the count for duplicate elements:

struct CountedSet<Element: Hashable>: Collection, SetAlgebra {
    // implementation details

    /// Updates the count stored in the set for the given element,
    /// adding the element if necessary.
    ///
    /// - Parameter n: The new count for `element`. `n` must be greater
    ///   than or equal to zero.
    /// - Parameter element: The element to set the new count on.
    mutating func updateCount(_ n: Int, for element: Element)
}

extension CountedSet: ExpressibleByDictionaryLiteral {
    init(dictionaryLiteral elements: (Element, Int)...) {
        self.init()
        for (element, count) in elements {
            self.updateCount(count, for: element)
        }
    }
}