How exactly do interfaces work in Go?

Question

After reading the spec, and the "Effective Go" section on them, I still don't quite understand how interfaces work in Go.

Like, where do you define them? How does interface enforcement work? And is there a way to specify somewhere that an object implements an interface, as opposed to simply defining the methods in the interface?

Apologies for the beginner question; but I really am struggling to understand this.

Answer 1

Basically, you define an interface like this:

type InterfaceNameHere interface {
    MethodA(*arg1, *arg2)
    MethodB(*arg3)
}

That particular interface definition requires anything which implements the interface to have both a MethodA method that takes 2 arguments, and a MethodB method that takes 1 argument.

Once you've defined it, Go will automatically check when you try to use something where a certain interface is required, whether the thing you're using satisfies that interface. You don't have to explicitly state that a given thing satisfies a given interface, it's just automatically checked when you try to utilize something in a scenario where it's expected to satisfy it.

Answer 2

There are some good posts on interfaces over at Russ Cox and Ian Lance Taylor's blog which i recommend checking out. They'll probably cover your questions and more ...

I think a good conceptual example is the net package. There you'll find a connections interface(Conn), which is implemented by the TCPConn, the UnixConn, and the UDPConn. The Go pkg source is probably the best documentation for the Go language.