How setting 'nil' as a value of [String: String] dictionary is valid?

Question

I'm curious why this snippet works correctly in Playground:

var dict = [String: String]()
dict["key"] = nil
let value = dict["key"]

we can see that I declared Dictionary with non optional values, after checking it in Playground it works similarly to dictionary declared as [String: String?]

My question is where is the difference in terms of setting nil value between dictionary declared as [String: String] and [String: String?] ?

Answer 1

dict["key"] = nil is a shorthand to removing the key from the dictionary (same as using dict.removeValue(forKey: "key")). If there was a value under the "key" key, after this line the whole entry is removed from the dictionary (both the key and the value).

Read the subscripts docs to learn more:

If you assign nil as the value for the given key, the dictionary removes that key and its associated value.

In the following example, the key-value pair for the key "Aquamarine" is removed from the dictionary by assigning nil to the key-based subscript.

hues["Aquamarine"] = nil
print(hues)
// Prints "["Coral": 18, "Heliotrope": 296, "Cerise": 330]"

let value = dict["key"] gets the value for the key, and by definition returns nil if there is no entry for the given key (which is in your case).

According to docs, subscript returns either the value, or nil, if the key is not in the dictionary:

The value associated with key if key is in the dictionary; otherwise, nil.