Kotlin: why do I need to initialize a var with custom getter?

Question

Why do I need to initialize a var with a custom getter, that returns a constant?

var greeting: String // Property must be initialized  get() = "hello" 

I don't need initialization when I make greeting read-only (val)

Answer 1

Reason behind this is Backing field. When you create val with custom getter that does not use field identifier to access its value, then backing field is not generated.

val greeting: String     get() = "hello" 

If you do, then backing field is generated and needs to be initialized.

val greeting: String // Property must be initialized     get() = field 

Now with var. Since backing filed is generated by default, it must be initialized.

var greeting: String // Property must be initialized     get() = "hello" 

For this to work for var without initialization, you must provide a custom setter to prevent generation of backing field. For example:

var storage: String = "" var greeting: String     get() = "hello"     set(value) { storage = value} 

Answer 2

Your code does not have a custom setter, so it is equivalent to:

var greeting: String     get() = "hello"     set(v) {field = v}  // Generated by default 

The default implementation of set uses field, so you've got to initialise it.

By the same logic, you don't have to init the field if nether your set nor get use it (which means they are both custom):

var greeting: String  // no `field` associated!     get() = "hello"     set(v) = TODO()