Hands-On System Programming with Go

Interface assertion

Assertion can also be done from one interface to another. Imagine having two different interfaces:

type Fooer interface {    Foo()}type Barer interface {    Bar()}

Let's define a type that implements one of them and another that implements both:

type A intfunc (A) Foo() {}type B intfunc (B) Bar() {}func (B) Foo() {}

If we define a new variable for the first interface, the assertion to the second is going to be successful only if the underlying value has a type that implements both; otherwise, it's going to fail:

func main() {    var a Fooer       a = A(0)    v, ok := a.(Barer)    fmt.Println(v, ok)    a = B(0)     v, ok = a.(Barer)    fmt.Println(v, ok)}

A full example is available here: https://play.golang.org/p/bX2rnw5pRXJ.

A use case scenario ...