A type that can be initialized using a dictionary literal.
SDK
- Xcode 8.0+
Framework
- Swift Standard Library
Declaration
protocol ExpressibleByDictionaryLiteral
Overview
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(dictionary initializer. The following example shows the dictionary literal initializer for a hypothetical Counted 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)
}
}
}