What is a nested class or nested struct?

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

Nested types – i.e. nested structs and nested classes – are a useful way of organizing your code, and perhaps even restricting what others can do with it. In essence, a nested type is just one data type defined inside another, like this:

struct Deck {
    struct Card {

    }
}

Now rather than creating a new Card struct you would instead create a new Deck.Card because one is inside the other.

This approach is useful for organizing larger applications, because you might need custom types that are applicable only in certain places – exposing them elsewhere might cause problems.

If you want to re-enforce this, Swift lets you mark the inner type (Card above) as being private, which means it can only be used inside the Deck class – code from outside Deck can’t even see it.