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# 2.5 面向对象
前面两章我们介绍了函数和struct，那你是否想过函数当作struct的字段一样来处理呢？今天我们就讲解一下函数的另一种形态，带有接收者的函数，我们称为`method`
## method
现在假设有这么一个场景，你定义了一个struct叫做长方形，你现在想要计算他的面积，那么按照我们一般的思路应该会用下面的方式来实现
package main
import "fmt"
type Rectangle struct {
width, height float64
}
func area(r Rectangle) float64 {
return r.width*r.height
}
func main() {
r1 := Rectangle{12, 2}
r2 := Rectangle{9, 4}
fmt.Println("Area of r1 is: ", area(r1))
fmt.Println("Area of r2 is: ", area(r2))
}
这段代码可以计算出来长方形的面积，但是area()不是作为Rectangle的方法实现的（类似面向对象里面的方法），而是将Rectangle的对象（如r1,r2）作为参数传入函数计算面积的。
这样实现当然没有问题咯，但是当需要增加圆形、正方形、五边形甚至其它多边形的时候，你想计算他们的面积的时候怎么办啊？那就只能增加新的函数咯，但是函数名你就必须要跟着换了，变成`area_rectangle, area_circle, area_triangle...`
像下图所表示的那样， 椭圆代表函数, 而这些函数并不从属于struct(或者以面向对象的术语来说，并不属于class)，他们是单独存在于struct外围，而非在概念上属于某个struct的。
![](images/2.5.rect_func_without_receiver.png?raw=true)
图2.8 方法和struct的关系图
很显然，这样的实现并不优雅，并且从概念上来说"面积"是"形状"的一个属性，它是属于这个特定的形状的，就像长方形的长和宽一样。
基于上面的原因所以就有了`method`的概念，`method`是附属在一个给定的类型上的，他的语法和函数的声明语法几乎一样，只是在`func`后面增加了一个receiver(也就是method所依从的主体)。
用上面提到的形状的例子来说，method `area()` 是依赖于某个形状(比如说Rectangle)来发生作用的。Rectangle.area()的发出者是Rectangle， area()是属于Rectangle的方法，而非一个外围函数。
更具体地说，Rectangle存在字段length 和 width, 同时存在方法area(), 这些字段和方法都属于Rectangle。
用Rob Pike的话来说就是：
>"A method is a function with an implicit first argument, called a receiver."
method的语法如下：
func (r ReceiverType) funcName(parameters) (results)
下面我们用最开始的例子用method来实现：
package main
import (
"fmt"
"math"
)
type Rectangle struct {
width, height float64
}
type Circle struct {
radius float64
}
func (r Rectangle) area() float64 {
return r.width*r.height
}
func (c Circle) area() float64 {
return c.radius * c.radius * math.Pi
}
func main() {
r1 := Rectangle{12, 2}
r2 := Rectangle{9, 4}
c1 := Circle{10}
c2 := Circle{25}
fmt.Println("Area of r1 is: ", r1.area())
fmt.Println("Area of r2 is: ", r2.area())
fmt.Println("Area of c1 is: ", c1.area())
fmt.Println("Area of c2 is: ", c2.area())
}
在使用method的时候重要注意几点
- 虽然method的名字一模一样，但是如果接收者不一样，那么method就不一样
- method里面可以访问接收者的字段
- 调用method通过`.`访问，就像struct里面访问字段一样
图示如下:
![](images/2.5.shapes_func_with_receiver_cp.png?raw=true)
图2.9 不同struct的method不同
在上例，method area() 分别属于Rectangle和Circle， 于是他们的 Receiver 就变成了Rectangle 和 Circle, 或者说，这个area()方法 是由 Rectangle/Circle 发出的。
>值得说明的一点是，图示中method用虚线标出，意思是此处方法的Receiver是以值传递，而非引用传递，是的，Receiver还可以是指针, 两者的差别在于, 指针作为Receiver会对实例对象的内容发生操作,而普通类型作为Receiver仅仅是以副本作为操作对象,并不对原实例对象发生操作。后文对此会有详细论述。
那是不是method只能作用在struct上面呢？当然不是咯，他可以定义在任何你自定义的类型、内置类型、struct等各种类型上面。这里你是不是有点迷糊了，什么叫自定义类型，自定义类型不就是struct嘛，不是这样的哦，struct只是自定义类型里面一种比较特殊的类型而已，还有其他自定义类型申明，可以通过如下这样的申明来实现。
type typeName typeLiteral
请看下面这个申明自定义类型的代码
type ages int
type money float32
type months map[string]int
m := months {
"January":31,
"February":28,
...
"December":31,
}
看到了吗？简单的很吧，这样你就可以在自己的代码里面定义有意义的类型了，实际上只是一个定义了一个别名,有点类似于c中的typedef，例如上面ages替代了int
好了，让我们回到`method`
你可以在任何的自定义类型中定义任意多的`method`，接下来让我们看一个复杂一点的例子
package main
import "fmt"
const(
WHITE = iota
BLACK
BLUE
RED
YELLOW
)
type Color byte
type Box struct {
width, height, depth float64
color Color
}
type BoxList []Box //a slice of boxes
func (b Box) Volume() float64 {
return b.width * b.height * b.depth
}
func (b *Box) SetColor(c Color) {
b.color = c
}
func (bl BoxList) BiggestColor() Color {
v := 0.00
k := Color(WHITE)
for _, b := range bl {
if bv := b.Volume(); bv > v {
v = bv
k = b.color
}
}
return k
}
func (bl BoxList) PaintItBlack() {
for i, _ := range bl {
bl[i].SetColor(BLACK)
}
}
func (c Color) String() string {
strings := []string {"WHITE", "BLACK", "BLUE", "RED", "YELLOW"}
return strings[c]
}
func main() {
boxes := BoxList {
Box{4, 4, 4, RED},
Box{10, 10, 1, YELLOW},
Box{1, 1, 20, BLACK},
Box{10, 10, 1, BLUE},
Box{10, 30, 1, WHITE},
Box{20, 20, 20, YELLOW},
}
fmt.Printf("We have %d boxes in our set\n", len(boxes))
fmt.Println("The volume of the first one is", boxes[0].Volume(), "cm³")
fmt.Println("The color of the last one is",boxes[len(boxes)-1].color.String())
fmt.Println("The biggest one is", boxes.BiggestColor().String())
fmt.Println("Let's paint them all black")
boxes.PaintItBlack()
fmt.Println("The color of the second one is", boxes[1].color.String())
fmt.Println("Obviously, now, the biggest one is", boxes.BiggestColor().String())
}
上面的代码通过const定义了一些常量，然后定义了一些自定义类型
- Color作为byte的别名
- 定义了一个struct:Box，含有三个长宽高字段和一个颜色属性
- 定义了一个slice:BoxList，含有Box
然后以上面的自定义类型为接收者定义了一些method
- Volume()定义了接收者为Box，返回Box的容量
- SetColor(c Color)，把Box的颜色改为c
- BiggestColor()定在在BoxList上面，返回list里面容量最大的颜色
- PaintItBlack()把BoxList里面所有Box的颜色全部变成黑色
- String()定义在Color上面，返回Color的具体颜色(字符串格式)
上面的代码通过文字描述出来之后是不是很简单？我们一般解决问题都是通过问题的描述，去写相应的代码实现。
### 指针作为receiver
现在让我们回过头来看看SetColor这个method，它的receiver是一个指向Box的指针，是的，你可以使用*Box。想想为啥要使用指针而不是Box本身呢？
我们定义SetColor的真正目的是想改变这个Box的颜色，如果不传Box的指针，那么SetColor接受的其实是Box的一个copy，也就是说method内对于颜色值的修改，其实只作用于Box的copy，而不是真正的Box。所以我们需要传入指针。
这里可以把receiver当作method的第一个参数来看，然后结合前面函数讲解的传值和传引用就不难理解
这里你也许会问了那SetColor函数里面应该这样定义`*b.Color=c`,而不是`b.Color=c`,因为我们需要读取到指针相应的值。
你是对的，其实Go里面这两种方式都是正确的，当你用指针去访问相应的字段时(虽然指针没有任何的字段)，Go知道你要通过指针去获取这个值，看到了吧，Go的设计是不是越来越吸引你了。
也许细心的读者会问这样的问题，PaintItBlack里面调用SetColor的时候是不是应该写成`(&bl[i]).SetColor(BLACK)`，因为SetColor的receiver是*Box，而不是Box。
你又说对的，这两种方式都可以，因为Go知道receiver是指针，他自动帮你转了。
也就是说：
>如果一个method的receiver是*T,你可以在一个T类型的实例变量V上面调用这个method，而不需要&V去调用这个method
类似的
>如果一个method的receiver是T，你可以在一个*T类型的变量P上面调用这个method，而不需要 *P去调用这个method
所以，你不用担心你是调用的指针的method还是不是指针的method，Go知道你要做的一切，这对于有多年C/C++编程经验的同学来说，真是解决了一个很大的痛苦。
### method继承
前面一章我们学习了字段的继承，那么你也会发现Go的一个神奇之处，method也是可以继承的。如果匿名字段实现了一个method，那么包含这个匿名字段的struct也能调用该method。让我们来看下面这个例子
package main
import "fmt"
type Human struct {
name string
age int
phone string
}
type Student struct {
Human //匿名字段
school string
}
type Employee struct {
Human //匿名字段
company string
}
//在human上面定义了一个method
func (h *Human) SayHi() {
fmt.Printf("Hi, I am %s you can call me on %s\n", h.name, h.phone)
}
func main() {
mark := Student{Human{"Mark", 25, "222-222-YYYY"}, "MIT"}
sam := Employee{Human{"Sam", 45, "111-888-XXXX"}, "Golang Inc"}
mark.SayHi()
sam.SayHi()
}
### method重写
上面的例子中，如果Employee想要实现自己的SayHi,怎么办？简单，和匿名字段冲突一样的道理，我们可以在Employee上面定义一个method，重写了匿名字段的方法。请看下面的例子
package main
import "fmt"
type Human struct {
name string
age int
phone string
}
type Student struct {
Human //匿名字段
school string
}
type Employee struct {
Human //匿名字段
company string
}
//Human定义method
func (h *Human) SayHi() {
fmt.Printf("Hi, I am %s you can call me on %s\n", h.name, h.phone)
}
//Employee的method重写Human的method
func (e *Employee) SayHi() {
fmt.Printf("Hi, I am %s, I work at %s. Call me on %s\n", e.name,
e.company, e.phone) //Yes you can split into 2 lines here.
}
func main() {
mark := Student{Human{"Mark", 25, "222-222-YYYY"}, "MIT"}
sam := Employee{Human{"Sam", 45, "111-888-XXXX"}, "Golang Inc"}
mark.SayHi()
sam.SayHi()
}
上面的代码设计的是如此的美妙，让人不自觉的为Go的设计惊叹！
通过这些内容，我们可以设计出基本的面向对象的程序了，但是Go里面的面向对象是如此的简单，没有任何的私有、公有关键字，通过大小写来实现(大写开头的为公有，小写开头的为私有)，方法也同样适用这个原则。
## links
* [目录](<preface.md>)
* 上一章: [struct类型](<02.4.md>)
* 下一节: [interface](<02.6.md>)
# Object-oriented
We talked about functions and structs in the last two sections, but did you ever consider using functions as fields of a struct? In this section, I will introduce you to another form of method that has a receiver, which is called `method`.
## method
Suppose you define a "rectangle" struct and you want to calculate its area. We'd typically use the following code to achieve this goal.
package main
import "fmt"
type Rectangle struct {
width, height float64
}
func area(r Rectangle) float64 {
return r.width*r.height
}
func main() {
r1 := Rectangle{12, 2}
r2 := Rectangle{9, 4}
fmt.Println("Area of r1 is: ", area(r1))
fmt.Println("Area of r2 is: ", area(r2))
}
The above example can calculate a rectangle's area. We use the function called `area`, but it's not a method of the rectangle struct (like class methods in classic object-oriented languages). The function and struct are two independent things as you may notice.
It's not a problem so far. However, if you also have to calculate the area of a circle, square, pentagon, or any other kind of shape, you are going to need to add additional functions with very similar names.
![](images/2.5.rect_func_without_receiver.png?raw=true)
Figure 2.8 Relationship between function and struct
Obviously that's not cool. Also, the area should really be the property of a circle or rectangle.
For those reasons, we have the `method` concept. `method` is affiliated with type. It has the same syntax as functions do except for an additional parameter after the `func` keyword called the `receiver`, which is the main body of that method.
Using the same example, `Rectangle.area()` belongs directly to rectangle, instead of as a peripheral function. More specifically, `length`, `width` and `area()` all belong to rectangle.
As Rob Pike said.
"A method is a function with an implicit first argument, called a receiver."
Syntax of method.
func (r ReceiverType) funcName(parameters) (results)
Let's change our example using `method` instead.
package main
import (
"fmt"
"math"
)
type Rectangle struct {
width, height float64
}
type Circle struct {
radius float64
}
func (r Rectangle) area() float64 {
return r.width*r.height
}
func (c Circle) area() float64 {
return c.radius * c.radius * math.Pi
}
func main() {
r1 := Rectangle{12, 2}
r2 := Rectangle{9, 4}
c1 := Circle{10}
c2 := Circle{25}
fmt.Println("Area of r1 is: ", r1.area())
fmt.Println("Area of r2 is: ", r2.area())
fmt.Println("Area of c1 is: ", c1.area())
fmt.Println("Area of c2 is: ", c2.area())
}
Notes for using methods.
- If the name of methods are the same but they don't share the same receivers, they are not the same.
- Methods are able to access fields within receivers.
- Use `.` to call a method in the struct, the same way fields are called.
![](images/2.5.shapes_func_with_receiver_cp.png?raw=true)
Figure 2.9 Methods are different in different structs
In the example above, the area() methods belong to both Rectangle and Circle respectively, so the receivers are Rectangle and Circle.
One thing that's worth noting is that the method with a dotted line means the receiver is passed by value, not by reference. The difference between them is that a method can change its receiver's values when the receiver is passed by reference, and it gets a copy of the receiver when the receiver is passed by value.
Can the receiver only be a struct? Of course not. Any type can be the receiver of a method. You may be confused about customized types. Struct is a special kind of customized type -there are more customized types.
Use the following format to define a customized type.
type typeName typeLiteral
Examples of customized types:
type ages int
type money float32
type months map[string]int
m := months {
"January":31,
"February":28,
...
"December":31,
}
I hope that you know how to use customized types now. Similar to `typedef` in C, we use `ages` to substitute `int` in the above example.
Let's get back to talking about `method`.
You can use as many methods in custom types as you want.
package main
import "fmt"
const(
WHITE = iota
BLACK
BLUE
RED
YELLOW
)
type Color byte
type Box struct {
width, height, depth float64
color Color
}
type BoxList []Box //a slice of boxes
func (b Box) Volume() float64 {
return b.width * b.height * b.depth
}
func (b *Box) SetColor(c Color) {
b.color = c
}
func (bl BoxList) BiggestsColor() Color {
v := 0.00
k := Color(WHITE)
for _, b := range bl {
if b.Volume() > v {
v = b.Volume()
k = b.color
}
}
return k
}
func (bl BoxList) PaintItBlack() {
for i, _ := range bl {
bl[i].SetColor(BLACK)
}
}
func (c Color) String() string {
strings := []string {"WHITE", "BLACK", "BLUE", "RED", "YELLOW"}
return strings[c]
}
func main() {
boxes := BoxList {
Box{4, 4, 4, RED},
Box{10, 10, 1, YELLOW},
Box{1, 1, 20, BLACK},
Box{10, 10, 1, BLUE},
Box{10, 30, 1, WHITE},
Box{20, 20, 20, YELLOW},
}
fmt.Printf("We have %d boxes in our set\n", len(boxes))
fmt.Println("The volume of the first one is", boxes[0].Volume(), "cm³")
fmt.Println("The color of the last one is",boxes[len(boxes)-1].color.String())
fmt.Println("The biggest one is", boxes.BiggestsColor().String())
fmt.Println("Let's paint them all black")
boxes.PaintItBlack()
fmt.Println("The color of the second one is", boxes[1].color.String())
fmt.Println("Obviously, now, the biggest one is", boxes.BiggestsColor().String())
}
We define some constants and customized types.
- Use `Color` as alias of `byte`.
- Define a struct `Box` which has fields height, width, length and color.
- Define a struct `BoxList` which has `Box` as its field.
Then we defined some methods for our customized types.
- Volume() uses Box as its receiver and returns volume of Box.
- SetColor(c Color) changes Box's color.
- BiggestsColor() returns the color which has the biggest volume.
- PaintItBlack() sets color for all Box in BoxList to black.
- String() use Color as its receiver, returns the string format of color name.
Is it much clearer when we use words to describe our requirements? We often write our requirements before we start coding.
### Use pointer as receiver
Let's take a look at `SetColor` method. Its receiver is a pointer of Box. Yes, you can use `*Box` as a receiver. Why do we use a pointer here? Because we want to change Box's color in this method. Thus, if we don't use a pointer, it will only change the value inside a copy of Box.
If we see that a receiver is the first argument of a method, it's not hard to understand how it works.
You might be asking why we aren't using `(*b).Color=c` instead of `b.Color=c` in the SetColor() method. Either one is OK here because Go knows how to interpret the assignment. Do you think Go is more fascinating now?
You may also be asking whether we should use `(&bl[i]).SetColor(BLACK)` in `PaintItBlack` because we pass a pointer to `SetColor`. Again, either one is OK because Go knows how to interpret it!
### Inheritance of method
We learned about inheritance of fields in the last section. Similarly, we also have method inheritance in Go. If an anonymous field has methods, then the struct that contains the field will have all the methods from it as well.
package main
import "fmt"
type Human struct {
name string
age int
phone string
}
type Student struct {
Human // anonymous field
school string
}
type Employee struct {
Human
company string
}
// define a method in Human
func (h *Human) SayHi() {
fmt.Printf("Hi, I am %s you can call me on %s\n", h.name, h.phone)
}
func main() {
mark := Student{Human{"Mark", 25, "222-222-YYYY"}, "MIT"}
sam := Employee{Human{"Sam", 45, "111-888-XXXX"}, "Golang Inc"}
mark.SayHi()
sam.SayHi()
}
### Method overload
If we want Employee to have its own method `SayHi`, we can define a method that has the same name in Employee, and it will hide `SayHi` in Human when we call it.
package main
import "fmt"
type Human struct {
name string
age int
phone string
}
type Student struct {
Human
school string
}
type Employee struct {
Human
company string
}
func (h *Human) SayHi() {
fmt.Printf("Hi, I am %s you can call me on %s\n", h.name, h.phone)
}
func (e *Employee) SayHi() {
fmt.Printf("Hi, I am %s, I work at %s. Call me on %s\n", e.name,
e.company, e.phone) //Yes you can split into 2 lines here.
}
func main() {
mark := Student{Human{"Mark", 25, "222-222-YYYY"}, "MIT"}
sam := Employee{Human{"Sam", 45, "111-888-XXXX"}, "Golang Inc"}
mark.SayHi()
sam.SayHi()
}
You are able to write an Object-oriented program now, and methods use rule of capital letter to decide whether public or private as well.
## Links
- [Directory](preface.md)
- Previous section: [struct](02.4.md)
- Next section: [interface](02.6.md)