what does it mean assign “_” to a field in scala?

Question

I saw some scala code that assign \"_\" to a field of class, what does it mean ? Thanks

private var tk: TaggedKey = _

Answer 1

It means: assign default value. Default value is defined as null, 0 or false depending on the target type.

It is described in 4.2 Variable Declarations and Definitions of the The Scala Language Specification:

A variable definition var x : T = _ can appear only as a member of a template. It introduces a mutable field with type T and a default initial value. The default value depends on the type T as follows:

0 - if Tis Int or one of its subrange types,

0L - if Tis Long,

0.0f - if Tis Float,

0.0d - if Tis Double,

false - if Tis Boolean,

() - if Tis Unit,

null - for all other types T.