What is the difference between an empty interface expression and a nil value in Go?

In Go language, an empty interface (interface{}) is a type that can hold a value of any type, including nil. However, the expression of an empty interface and the value nil are different things.

• var x interface{} — variable x of an empty interface type, which by default is equal to nil, meaning it contains neither type nor value.
• But if you assign some value to x, for example x = 5, then x is no longer nil, as it contains the type int and the value 5.

The difference is that in Go, an interface internally is represented by two pointers: one to the type and one to the value. An interface equals nil only if both pointers are nil.

Example:

var i interface{} // i == nil
var p *int = nil

 i = p // i != nil, because the type *int is present, even though the value is nil

fmt.Println(i == nil) // false

Thus, an empty interface can contain a value of nil of a specific type, but the interface itself will not be equal to nil. This is an important point when checking for nil in Go.