How Interfaces Are Represented Internally in Go (iface and eface)

Go represents interfaces internally using two distinct structures: iface for non-empty interfaces and eface for the empty interface{} type. An iface contains a pointer to an itab (interface table) describing the method set and a pointer to the data, while an eface contains only a pointer to the type description and the data.

// Simplified internal representation from cmd/compile/internal/abi-internal.md
// eface (empty interface): 2 words
type eface struct {
    typ  *type // Pointer to runtime type description
    data unsafe.Pointer // Data field (direct or indirect)
}

// iface (non-empty interface): 2 words
type iface struct {
    itab *itab // Pointer to interface table (method set + type)
    data unsafe.Pointer // Data field (direct or indirect)
}

Think of an interface as a universal adapter. The empty interface (eface) is a generic slot that just holds the item and a label saying what it is. A specific interface (iface) is a specialized slot that holds the item, a label, and a specific instruction manual (itab) on how to use that item's methods. This distinction allows Go to check if a value supports specific methods efficiently.