How are Go interfaces implemented internally and why do they matter for performance?

A Go interface value is a two-word struct: a type pointer and a data pointer. There are two variants in the runtime: iface (for interfaces with methods — has a pointer to the method table / itab) and eface (for the empty interface any — just type and data pointers).

// Empty interface (any / interface{})
// eface { type *_type, data unsafe.Pointer }
var v any = 42
// _type = pointer to int's type descriptor
// data = pointer to a heap-allocated int holding 42

// Interface with methods
// iface { itab *itab, data unsafe.Pointer }
// itab = {inter *interfacetype, type *_type, hash uint32, fun [...]uintptr}
// fun[] = the method dispatch table (virtual call table)

type Stringer interface { String() string }
type MyInt int
func (m MyInt) String() string { return fmt.Sprintf("%d", m) }

var s Stringer = MyInt(5) // iface{itab=*MyInt-Stringer-itab, data=ptr-to-5}
fmt.Println(s.String())    // dispatches via itab.fun[0]

// Type assertion — recovers the concrete type
if mi, ok := s.(MyInt); ok {
    fmt.Println(mi + 1)
}

// Type switch
switch t := v.(type) {
case int:    fmt.Println("int:", t)
case string: fmt.Println("string:", t)
default:     fmt.Println("unknown")
}

// Performance implication: values stored in interface often escape to heap
// Small values (pointer-sized or smaller) can be inlined into the data word
// Avoid interface{} in hot paths — use generics (Go 1.18+) instead

Interface comparison: two interface values are equal if and only if their type pointers are equal (same concrete type) and their data pointers point to equal values. Comparing interfaces with non-comparable concrete types (e.g., slices) panics at runtime.

Nil interface pitfall: (*MyType)(nil) stored in an interface is not a nil interface — the interface has a non-nil type pointer. Always return a nil interface (return nil) rather than a typed nil pointer when returning an interface from a function.

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