What is the difference between reference types and value types?

A reference stores an address in memory pointing to the data stored there. In fact, a variable holding a reference only owns this address, not the data itself. Assigning a reference type to a variable copies the reference, not the data. Changes to the data pointed to by the reference are reflected in all variables holding that reference.

A value type directly stores the data itself. Assigning a value type to a variable copies the data itself. Changes in one variable of a value type do not affect other variables that received this copy.

Attribute	Reference Type	Value Type
Storage	Reference to data	Data itself
Assignment	Copying the reference	Copying the data
Modification	Affects all references	Does not affect copies
Placement	Heap	Stack
Deallocation	ARC / Garbage collector	Automatically upon scope exit
Examples in Swift	class, closure	struct, enum, tuple, basic types (Int, Bool, String, etc.)

// Example of a reference type (class)
class ReferenceType {
    var value: Int
    init(value: Int) {
        self.value = value
    }
}

let ref1 = ReferenceType(value: 10)
let ref2 = ref1 // Reference is copied
ref2.value = 20 // Data pointed to by ref1 and ref2 is changed
print(ref1.value) // Output: 20
print(ref2.value) // Output: 20

// Example of a value type (struct)
struct ValueType {
    var value: Int
}

var val1 = ValueType(value: 10)
var val2 = val1 // Data is copied
val2.value = 20 // Only data in val2 is changed
print(val1.value) // Output: 10
print(val2.value) // Output: 20