Why does embedding an Interface in a struct cause the Interface methodset to be defined as nil pointer?

Question

I'm working through learning Go, and I've come across embedding Interfaces into structs in Go.

I understand the joys of Interfaces and their implementations, but I'm confused as the reasoning for the current execution of embedding one inside a struct.

When I embed an interface in a struct, the struct gains the methodset of the Interface and can now be used as a value of an Interface type variable, eg:

type Foo interface {
  SetBaz(baz) 
  GetBaz() baz
}

type Bar struct {
  Foo
}

So now we have a struct type Bar which embeds Foo. Because Bar embeds Foo, Bar now satisfies any receivers or arguments that require type Foo, even though Bar hasn't even defined them.

Trying to call Bar.GetBaz() causes the runtime error: panic: runtime error: invalid memory address or nil pointer dereference.

Why does Go define nil methods on a struct that embeds an interface instead of explicitly requiring those methods to be defined through the compiler?

Go doesn't define "nil methods" (there's isn't such a thing). You're calling a method on a nil interface. It works exactly the same as if you embed a pointer. — JimB

Elwinar Elwinar · Accepted Answer · 2016-01-12T21:55:19

You're wrong about the nil method, it's the interface embedded into the struct Bar that is nil. When you use the methods of the interface, that is this interface that is called. The trick allow you to override an interface method with our own.

To understand the usage and goals of embedding interfaces into struct, the best example is in the sort package:

type reverse struct {
    // This embedded Interface permits Reverse to use the methods of
    // another Interface implementation.
    Interface
}

// Less returns the opposite of the embedded implementation's Less method.
func (r reverse) Less(i, j int) bool {
    return r.Interface.Less(j, i)
}

// Reverse returns the reverse order for data.
func Reverse(data Interface) Interface {
    return &reverse{data}
}

Why does embedding an Interface in a struct cause the Interface methodset to be defined as nil pointer?

1 Answers