Kotlin Programming Cookbook

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The apply function is quite similar to the also function, but they have a subtle difference. To understand that, let's look at their implementation first:

• The also function:

public inline fun <T> T.also(block: (T) -> Unit): T { block(this); return this }

• The apply function:

public inline fun <T> T.apply(block: T.() -> Unit): T { block(); return this }

In also, the block is defined as (T) -> Unit, but it is defined as T.() -> Unit in apply(), which means there is an implicit this inside the apply block. However, to reference it in also, we need it.

So a code using also will look like this:

val result = Dog(12).also { it.age = 13 }

The same will look like this using apply:

val result2 =Dog(12).apply {age = 13 }

The age ...