src/internal/abi/iface.go

// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package abi

import (
	"internal/goarch"
	"unsafe"
)

// The first word of every non-empty interface type contains an *ITab.
// It records the underlying concrete type (Type), the interface type it
// is implementing (Inter), and some ancillary information.
//
// allocated in non-garbage-collected memory
type ITab struct {
	Inter *InterfaceType
	Type  *Type
	Hash  uint32     // copy of Type.Hash. Used for type switches.
	Fun   [1]uintptr // variable sized. fun[0]==0 means Type does not implement Inter.
}

// Size returns the size of the itab in memory.
func (it *ITab) Size() int {
	size := int(unsafe.Sizeof(ITab{}))
	if it.Fun[0] == 0 {
		return size
	}
	return size + (len(it.Inter.Methods)-1)*goarch.PtrSize
}

// EmptyInterface describes the layout of a "interface{}" or a "any."
// These are represented differently than non-empty interface, as the first
// word always points to an abi.Type.
type EmptyInterface struct {
	Type *Type
	Data unsafe.Pointer
}

// NonEmptyInterface describes the layout of an interface that contains any methods.
type NonEmptyInterface struct {
	ITab *ITab
	Data unsafe.Pointer
}

// CommonInterface describes the layout of both [EmptyInterface] and [NonEmptyInterface].
type CommonInterface struct {
	// Either an *ITab or a *Type, unexported to avoid accidental use.
	_ unsafe.Pointer

	Data unsafe.Pointer
}