结构体，方法和接口

package main

import "math"

//在 Go 语言中 interface resolution 是隐式的。如果传入的类型匹配接口需要的，则编译正确。
//Shape is implemented by anything that can tell us its Area
type Shape interface {
    Area() float64
}

//Rectangle has the dimensions of a rectangle
type Rectangle struct {
    Width  float64
    Height float64
}
//方法和函数很相似,但是方法是通过一个特定类型的实例调用的。
//函数可以随时被调用,比如Perimeter(rectangle Rectangle)。不像方法需要在某个事物上调用。


//声明方法的语法跟函数差不多，因为他们本身就很相似。唯一的不同是方法接收者的语法 func(receiverName ReceiverType) MethodName(args)。
//当方法被这种类型的变量调用时，数据的引用,通过变量 receiverName 获得。在其他许多编程语言中这些被隐藏起来并且通过 this 来获得接收者。
//把类型的第一个字母作为接收者变量是 Go 语言的一个惯例。 例如：r Rectangle

//Area returns the area of the rectangle
func (r Rectangle) Area() float64 {
    return r.Width * r.Height
}

//Perimeter returns the perimeter of the rectangle
func Perimeter(rectangle Rectangle) float64 {
    return 2 * (rectangle.Width + rectangle.Height)
}

//Circle represents a circle...
type Circle struct {
    Radius float64
}

//Area returns the ares of the circle
func (c Circle) Area() float64 {
    return math.Pi * c.Radius * c.Radius
}

//Triangle represents the dimensions of a triangle
type Triangle struct {
    Base   float64
    Height float64
}

//Area returns the area of the triangle
func (t Triangle) Area() float64 {
    return (t.Base * t.Height) * 0.5
}


package main

import "testing"

func TestPerimeter(t *testing.T) {
    rectangle := Rectangle{10.0, 10.0}
    got := Perimeter(rectangle)
    want := 40.0

    if got != want {
        t.Errorf("got %.2f want %.2f", got, want)
    }
}

func TestArea(t *testing.T) {
    areaTests := []struct {
        name    string
        shape   Shape
        hasArea float64
    }{
        {name: "Rectangle", shape: Rectangle{Width: 12, Height: 6}, hasArea: 72.0},
        {name: "Circle", shape: Circle{Radius: 10}, hasArea: 314.1592653589793},
        {name: "Triangle", shape: Triangle{Base: 12, Height: 6}, hasArea: 36.0},
    }

    for _, tt := range areaTests {
        got := tt.shape.Area()
        if got != tt.hasArea {
            t.Errorf("got %.2f want %.2f", got, tt.hasArea)
        }
    }
}