Borrowing and consuming, the ownership of values

Ownership tells the compiler how you plan to use a value.

borrowing - take a value temporarily consuming - take the value so the original owner cannot keep using it

import Foundation
 
func orderFood() {
    let food = Food(name: "🥐")✕'food' used after consume
 
    photograph(food)
 
    eat(food)ℹConsumed here
 
    photograph(food)ℹUsed here
}
 
func photograph(_ food: borrowing Food) { 
    print("Taking a photo of the \(food.name)...")
}
 
func eat(_ food: consuming Food) { 
    print("Eating \(food.name)...")
}
 
struct Food: ~Copyable { // Note: this is non-copyable
    let name: String
}

‘Photographing’ only borrows the food temporarily, but ‘eating’ the food means no one else can have access to it when we’re done.

With these simple keywords we can give the compiler enough information to catch mistakes at compile time instead of letting them become a 💥 at runtime.

Imagine in our code that instead of a 🥐 we were dealing with a file on disk.

Instead of photographing our file we want to read the contents of it, and, instead of eating, we’ll close access to the file.

Without this explicit ownership we might end up writing a function which tried to read the file after closing it, causing a failure for the user.

In the world where dealing with a file could lead to runtime failures for our users, this ownership can be moved to compile time with minimal change to our code. It also allows anyone reading the code to know how their value might be used.

We can add the consuming and borrowing keywords to our parameters to tell the compiler how we plan to use the value in our functions, giving the compiler enough information to give us compile time errors 👏🏻.

// The consume keyword

// Removing the ‘borrowing’ or ‘consuming’ keyword from the params gives a new compiler error

// Speed benefits, manifesto details

// arc overhead without this

—

https://github.com/swiftlang/swift/blob/main/docs/OwnershipManifesto.md

https://infinum.com/blog/swift-ownership/

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