15 Fake Inheritance in Golang

Golang 本身沒有 OOP，但可以透過 Struct Embedding/Anonymous Fields 的機制來模擬 OOP。也因此 Golang 可以模擬出多重繼承的效果。

為了避免混餚，建議 embedded struct 不要用 pointer。

單一繼承與 Override

example:

package main

import (
    "encoding/json"
    "log"
)

// Role ...
type Role struct {
    HP          float64
    MP          float64
    hiddenSkill string
}

func (r *Role) String() string {
    return "simple role"
}

// Move ...
func (r *Role) Move() (float64, float64) {
    return 1.0, 1.0
}

// Skill ...
func (r *Role) Skill() string {
    return r.hiddenSkill
}

// Dump ...
func (r *Role) Dump() string {
    bytes, _ := json.Marshal(r)
    return string(bytes)
}

// Magician ...
type Magician struct {
    Role
}

// NewMagician ...
func NewMagician() *Magician {
    return &Magician{Role{HP: 100.0, MP: 1000.0, hiddenSkill: "fireball"}}
}

// Move ...
func (m *Magician) Move() (float64, float64) {
    x, y := m.Role.Move()
    return 100 + x, 200 + y
}

func (m *Magician) String() string {
    return "magician"
}

func main() {
    role := &Role{}
    x, y := role.Move()
    log.Printf("%v: %s, skill: %q, move:(%g, %g)\n", role, role.Dump(), role.Skill(), x, y)

    magician := NewMagician()
    x, y = magician.Move()
    log.Printf("%v: %s, skill: %q, move:(%g, %g)\n", magician, magician.Dump(), magician.Skill(), x, y)

    magician.hiddenSkill += ";iceball"
    log.Printf("%v: %s, skill: %q, move:(%g, %g)\n", magician, magician.Dump(), magician.Skill(), x, y)
}

Output:

2018/03/29 11:17:55 simple role: {"HP":0,"MP":0}, skill: "", move:(1, 1)
2018/03/29 11:17:55 magician: {"HP":100,"MP":1000}, skill: "fireball", move:(101, 201)
2018/03/29 11:17:55 magician: {"HP":100,"MP":1000}, skill: "fireball;iceball", move:(101, 201)

說明：

1. 同 package 下，是可以存取 private 變數，如: magician.hiddenSkill += ";iceball"
2. 可以各自 implements interface，如: func (r *Role) String() string, func (m *Magician) String() string。如果 Magician 沒有 implements Stringer，則會用 Role 的 String()
3. 可以透過 m.Role 來達到存取父類別的效果，如: x, y := m.Role.Move()
4. 重新宣告與 embed struct 的 method，可以達到 override 效果，如: func (m *Magician) Move() (float64, float64) override func (r *Role) Move() (float64, float64)

多重繼承與 Ambiguous

example:

package main

import (
    "encoding/json"
    "log"
)

// Role ...
type Role struct {
    HP          float64
    MP          float64
    hiddenSkill string
}

func (r *Role) String() string {
    return "simple role"
}

// Move ...
func (r *Role) Move() (float64, float64) {
    return 1.0, 1.0
}

// Skill ...
func (r *Role) Skill() string {
    return r.hiddenSkill
}

// Dump ...
func (r *Role) Dump() string {
    bytes, _ := json.Marshal(r)
    return string(bytes)
}

// Flyer ...
type Flyer struct {
}

// Fly ...
func (f *Flyer) Fly() (float64, float64) {
    return 1000.0, 1000.0
}

// Skill ...
func (f *Flyer) Skill() string {
    return "fly"
}

// Bahamut ...
type Bahamut struct {
    Role
    Flyer
}

func (r *Bahamut) String() string {
    return "bahamut"
}

// NewBahamut ...
func NewBahamut() *Bahamut {
    return &Bahamut{Role{100000.0, 100000.0, "fireball"}, Flyer{}}
}

func main() {
    bahamut := NewBahamut()
    x, y := bahamut.Move()
    log.Printf("%v: %s, skill: %q, move:(%g, %g)\n", bahamut, bahamut.Dump(), bahamut.Skill(), x, y) // error: ambiguous selector bahamut.Skill
}

說明:

1. 因為 Role, Flyer 都有 Skill function，會不知道要取那一個. 因為必預要指定，如: bahamut.Role.Skill() or bahamut.Flyer.Skill()。或者 override Skill function. 如下：

    // Skill ...
    func (b *Bahamut) Skill() string {
        return strings.Join([]string{b.Role.Skill(), b.Flyer.Skill()}, ";")
    }