Inline Classes

With Kotlin 1.3, inline isn’t just for functions anymore!

What?

In the preceding chapter, we saw inline applied to functions. This says “instead of making the actual function call, put the code for the function ‘inline’ where the call is being made”.

Kotlin 1.3 added support for inline applied to classes:

inline class InvoiceLineItemKey(val key: String)

An inline class is only allowed to have one property, and it must appear in the constructor. They are allowed to have functions and properties with custom accessors (but no backing fields).

When the code is compiled — assuming that there are no compile errors — all occurrences of this inline class are replaced by references to the one-and-only property of that class, and all references to function calls are replaced with inline representations of those functions.