父类:
1. 把要重载的方法用函数式参数存起来(充当虚表)
2. 调用方法从虚表查找实现, 没有的话就给个默认实现
3. 调用实现方法

子类:
1. 继承父类方法
2. 新增一个构造函数, 用于修改虚表, 覆盖父类实现
3. 创建对象并调用构造函数, 就可以愉快地玩耍了

package extension

import (
    "fmt"
    "testing"
)

type Pet struct {
    speakHandle func()
    speakToHandle func(string)
}

func (p *Pet) Speak() {
    if p.speakHandle == nil {
        p.speakHandle = func() {
            fmt.Print("...")
        }
    }
    p.speakHandle()
}

func (p *Pet) SpeakTo(host string) {
    if p.speakToHandle == nil {
        p.speakToHandle = func(_host string) {
            p.Speak()
            fmt.Println(_host)
        }
    }
    p.speakToHandle(host)
}

type Dog struct {
    Pet
}

func (d *Dog) Ctor() *Dog {
    overrideSpeakHandle(d)
    //overrideSpeakToHandle(d)
    return d
}

func overrideSpeakHandle(d *Dog) {
    if d.speakHandle == nil {
        d.speakHandle = func() {
            fmt.Print("Wang!")
        }
    }
}

func overrideSpeakToHandle(d *Dog) {
    if d.speakToHandle == nil {
        d.speakToHandle = func(_host string) {
            d.Speak()
            fmt.Println(_host + "...")
        }
    }
}

func TestDog(t *testing.T) {
    pet := Pet{}
    dog := new(Dog).Ctor()
    pet.SpeakTo("Xu")
    dog.SpeakTo("Xu")
}