How exactly does the "let" keyword work in Swift?

Question

I've read this simple explanation in the guide:

The value of a constant doesn’t need to be known at compile time, but you must assign it a value exactly once.

But I want a little more detail than this. If the constant references an object, can I still modify its properties? If it references a collection, can I add or remove elements from it? I come from a C# background; is it similar to how readonly works (apart from being able to use it in method bodies), and if it's not, how is it different?

Answer 1

let is a little bit like a const pointer in C. If you reference an object with a let, you can change the object's properties or call methods on it, but you cannot assign a different object to that identifier.

let also has implications for collections and non-object types. If you reference a struct with a let, you cannot change its properties or call any of its mutating func methods.

Using let/var with collections works much like mutable/immutable Foundation collections: If you assign an array to a let, you can't change its contents. If you reference a dictionary with let, you can't add/remove key/value pairs or assign a new value for a key — it's truly immutable. If you want to assign to subscripts in, append to, or otherwise mutate an array or dictionary, you must declare it with var.

(Prior to Xcode 6 beta 3, Swift arrays had a weird mix of value and reference semantics, and were partially mutable when assigned to a let -- that's gone now.)

Answer 2

Let

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Swift uses two basic techniques to store values for a programmer to access by using a name: let and var. Use let if you're never going to change the value associated with that name. Use var if you expect for that name to refer to a changing set of values.

let a = 5  // This is now a constant. "a" can never be changed. var b = 2  // This is now a variable. Change "b" when you like. 

The value that a constant refers to can never be changed, however the thing that a constant refers to can change if it is an instance of a class.

let a = 5 let b = someClass() a = 6  // Nope. b = someOtherClass()  // Nope. b.setCookies( newNumberOfCookies: 5 )  // Ok, sure. 

Let and Collections

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When you assign an array to a constant, elements can no longer be added or removed from that array. However, the value of any of that array's elements may still be changed.

let a = [1, 2, 3] a.append(4)  // This is NOT OK. You may not add a new value. a[0] = 0     // This is OK. You can change an existing value. 

A dictionary assigned to a constant can not be changed in any way.

let a = [1: "Awesome", 2: "Not Awesome"] a[3] = "Bogus"             // This is NOT OK. You may not add new key:value pairs. a[1] = "Totally Awesome"   // This is NOT OK. You may not change a value. 

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That is my understanding of this topic. Please correct me where needed. Excuse me if the question is already answered, I am doing this in part to help myself learn.