Value vs Reference Type in Swift

playground.swift

//: Playground - noun: a place where people can play

// Value Type

struct StructA {
  var field1: String = "Default Struct Value"
  let field2: Int = 1
}

let constantStruct = StructA()
var variableStruct = constantStruct

print(variableStruct.field1) // Same values with the constant value

variableStruct.field1 = "Changed mutable member of a mutable value type" // copy-on-write
print(constantStruct.field1) // Original value of the value type is not affected because of copy characteristic on value types
print(variableStruct.field1)


// Reference Type

class ClassA {
  var field1: String = "Default Class Value"
}

let constantClass = ClassA()
var variableClass = constantClass

print(variableClass.field1) // Same values with the constant value

variableClass.field1 = "Changed mutable member of a mutable reference type"
print(constantClass.field1) // Original value of the constant type is also changed since they share the same references
print(variableClass.field1)

constantClass.field1 = "Assigned a new value to a mutable member of an `immutable` reference type" // Mutating members of an `immutable` reference type is still allowed eventhough they are defined as `let`
print(constantClass.field1)
print(variableClass.field1) // Value is updated for the reference as well. They always share same pointers.



/* Compile-time errors for assignments. Following assignments are not allowed. That is why using structs are safer than classes depending on the context.
 It prevents side-effects and generates more compile time error for early recovery.
*/

//constantStruct.field1 = "Cannot assign a value to a mutable member of a immutable value type"
//constantStruct.field2 = "Cannot assign a value to a immutable member of a immutable value type"
//
//constantClass = variableClass