What is the internal two-word structure of a Go interface value?

Internally, every non-empty interface value is a two-word pair stored on the stack (or heap if it escapes):

type Animal interface { Sound() string }

type Dog struct{ Name string }
func (d Dog) Sound() string { return "Woof" }

var a Animal = Dog{Name: "Rex"}
// Memory layout of 'a':
// word1 → itab{itype=Animal, ctype=Dog, hash=..., fun=[Dog.Sound]}
// word2 → *Dog{Name: "Rex"}  (heap-allocated copy)

// Nil interface — BOTH words are nil
var a2 Animal // nil interface: word1=nil, word2=nil
fmt.Println(a2 == nil) // true

// Non-nil interface holding nil pointer — word1 is non-nil!
var d *Dog
a = d  // word1 → itab{ctype=*Dog,...}, word2 → nil
fmt.Println(a == nil) // FALSE — classic trap

// Inspect with reflect
import "reflect"
v := reflect.ValueOf(a)
fmt.Println(v.IsNil())  // true  — the *Dog data pointer is nil
fmt.Println(a == nil)   // false — the interface itself is non-nil

The empty interface any (alias for interface{}) uses a simpler two-word layout called eface: word1 is a plain type pointer (no method table), word2 is the data pointer. Non-empty interfaces use iface with the full itab. The itab is cached globally per (interface type, concrete type) pair, so interface assignment is effectively free after the first occurrence.

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