Value and reference types

The basic data types of Swift, such as Int, Double, and Bool, are said to be value types. This means that, when passing a value to a function (including assignment of variables and constants), it is copied into its new location:

var x1 = 1 
var y1 = x1 
y1 = 2 
x1 == 1 // true 

However, this concept extends to String, Array, Dictionary, and many other objects that, in some languages, notably Objective C, are passed by reference. Passing by reference means that we pass a pointer to the actual object itself, as an argument, rather than just copy its value:

var referenceObject1 = someValue 
var referenceObject2 = referenceObject1 
referenceObject2 = someNewValue 

These two variables now point to the same instance.

While this pattern is frequently desirable, it does leave a lot of variables sharing the same data--if you change one, you change the others. And that's a great source of bugs...