Go Interfaces Embedding

Go Interfaces & Embedding

Interface satisfaction is implicit and structural. Embedding promotes the inner type's method set; the receiver stays the inner type. Composition, not inheritance — no virtual dispatch.

Type switch vs assertion

switch t := v.(type) {
case int:   use(t)      // t is int
case *Foo:  use(t)      // t is *Foo
default:                // t keeps interface type
}

s, ok := v.(string)     // comma-ok avoids panic

Bare v.(string) panics on mismatch.

Compile-time interface compliance

var _ json.Marshaler = (*RawMessage)(nil)

Breaks the build if *RawMessage stops implementing Marshaler. Use when no other static conversion in the package already proves it.

Embedding: method promotion

type ReadWriter struct {
    *Reader
    *Writer
}
rw.Read(p)    // dispatches to (*Reader).Read; receiver is rw.Reader

The promoted method's receiver is the embedded value. An inner method calling m.Other() sees the inner Other, not any outer shadow — you cannot override polymorphically, only shadow at the outer level.

Name collisions

Access embedded fields by type name (minus package): j.Logger.

Interface embedding

type ReadWriter interface {
    Reader
    Writer
}

Union of method sets. Only between interfaces — cannot embed an interface in a struct to declare required methods.

Receivers and interfaces

A T value does not satisfy an interface whose method uses a pointer receiver.

Pitfalls

• Recursive String(): fmt.Sprintf("%s", x) inside (x T) String() recurses. Convert: fmt.Sprint([]int(x)).
• Discarding errors with _: fi, _ := os.Stat(p); fi.IsDir() nil-panics on failure.
• Assuming polymorphism: an inner method calling m.Other() calls the inner Other, not an outer shadow.
• Nil interface vs nil concrete: var p *T; var i Iface = p; i == nil is false — interface holds (type, value) and type is non-nil.
• Empty interface: prefer any (Go 1.18+) over interface{}; identical.