在 Golang 是否可以从父 struct 调用重写的方法？

小开

As you wrote, what Go has is not really inheritance, the method that allows inheritance like features is called Embedding.

http://golang.org/doc/effective_go.html#embedding

What it means basically is that the embedded struct has no idea that it is embedded, so you cannot override anything that is called from it. You can actually take the embedded struct and take a reference for it only from the embedding struct.

So your best way to do it is more or less like your second example - through some sort of dependency injection using interfaces. i.e - A has a reference to some interface that does the actual work, say worker, that writes to a file or whatever. Then when instantiating B, you also replace A's worker with another worker (you can do it even without embedding A of course). The A just does something like myWorker.Work() without caring what worker it is.

小开

package main


import (
"fmt"
)




//-- polymorphism in work


// children specification by methods signatures
// you should define overridable methods here
type AChildInterface interface {
Foo()
}


type A struct {
child AChildInterface
}


//-- /polymorphism in work




// hard A.Bar method
func (a *A) Bar() {
a.child.Foo() // Foo() will be overwritten = implemented in a specified child
}




//-- default implementations of changeable methods


type ADefaults struct{}


func (ad ADefaults) Foo() {
fmt.Println("A.Foo()")
}


//-- /default




//-- specified child


type B struct {
ADefaults // implement default A methods from ADefaults, not necessary in this example
}


// overwrite specified method
func (b B) Foo() {
fmt.Println("B.Foo()")
}


//-- /specified child


func main() {
a := A{ADefaults{}}
a.Bar()


// Golang-style inheritance = embedding child
b := A{B{}} // note: we created __Parent__ with specified __Child__ to change behavior
b.Bar()
}

Output:

A.Foo()
B.Foo()

小开

Recently I have a need to do this and the composition method proposed by OP works great.

I try to create another example to try to demonstrate the parent and child relationship and make it easier to read.

https://play.golang.org/p/9EmWhpyjHf:

package main


import (
"fmt"
"log"
)


type FruitType interface {
Wash() FruitType
Eat() string
}


type Fruit struct {
name  string
dirty bool
fruit FruitType
}


func (f *Fruit) Wash() FruitType {
f.dirty = false
if f.fruit != nil {
return f.fruit
}
return f
}
func (f *Fruit) Eat() string {
if f.dirty {
return fmt.Sprintf("The %s is dirty, wash it first!", f.name)
}
return fmt.Sprintf("%s is so delicious!", f.name)
}


type Orange struct {
*Fruit
}


func NewOrange() *Orange {
ft := &Orange{&Fruit{"Orange", true, nil}}
ft.fruit = ft
return ft
}
func NewApple() *Fruit {
ft := &Fruit{"apple", true, nil}
return ft
}


func (o *Orange) Eat() string {
return "The orange is so sour!"
}


func main() {
log.Println(NewApple().Eat())
log.Println(NewApple().Wash().Eat())
log.Println(NewOrange().Eat())
log.Println(NewOrange().Wash().Eat())
}

小开

Go does not support virtual method overriding. The design pattern you want to use is thus not directly supported by Go. It is considered bad practice because changing the implementation of A.Bar() would break all derived classes like B that assume A.Foo() will be called by A.Bar(). The design pattern you want to use will make your code brittle.

The way to do it in Go would be to define a Fooer interface with a Foo() method. A Fooer would be passed as argument to Bar() or stored in a field of A and called by A.Bar(). To change the Foo action, change the Fooer value. This is called composition, and it is much better than changing Foo action by inheritance and method overriding.

Here is an idiomatic way to do what you want to do in Go: https://play.golang.org/p/jJqXqmNUEHn. In this implementation, the Fooer is a member field of A that is initialized by a parameter of to the instance factory function NewA(). This design pattern is preferable when the Fooer doesn't change frequently during the lifetime of A. Otherwise, you may pass the Fooer as parameter of the Bar() method.

This is how we change the behavior of Foo() in Go. It is called composition because you change the behavior of Bar() by changing the instances composing A.

package main


import (
"fmt"
)


type Fooer interface {
Foo()
}


type A struct {
f Fooer
}


func (a *A) Bar() {
a.f.Foo()
}


func NewA(f Fooer) *A {
return &A{f: f}
}


type B struct {
}


func (b *B) Foo() {
fmt.Println("B.Foo()")
}


type C struct {
}


func (c *C) Foo() {
fmt.Println("C.Foo()")
}


func main() {
a := NewA(new(B))
a.Bar()


a.f = &C{}
a.Bar()
}

PS: Here is a possible implementation of the design pattern you wanted to implement for documentation purpose: https://play.golang.org/p/HugjIbYbout

小开

Been struggling with this myself. Found 2 solutions:

Idiomatic Go way: implement the common "method" as external function with interface as argument.

 package main


import "fmt"


// Fooer has to Foo
type Fooer interface {
Foo()
}


// Bar is a proxy, that calls Foo of specific instance.
func Bar(a Fooer) {
a.Foo()
}


//////////////////////////////////////////////////////////////////////
// usage


func main() {
b := &B{} // note it is a pointer
// also there's no need to specify values for default-initialized fields.
Bar(b) // prints: B.Foo()
}


//////////////////////////////////////////////////////////////////////
// implementation


// A is a "base class"
type A struct {
}


func (a *A) Foo() {
fmt.Println("A.Foo()")
}


// B overrides methods of A
type B struct {
A
}


func (b *B) Foo() {
fmt.Println("B.Foo()")
}

Try it on Go Playground: https://play.golang.org/p/2TbmHUs9_Dt

Similar to your second option: interface hackery. However, since Bar() is not specific to A (it is common to A and B), let's move it to base class, and hide implementation details and all dangerous stuff:

 package main


import "fmt"


//////////////////////////////////////////////////////////////////////
// Usage


func main() {
b := NewB()
b.Bar() // prints: B.Foo()


a := NewA()
a.Bar() // prints: A.Foo()
}


//////////////////////////////////////////////////////////////////////
// Implementation.


// aBase is common "ancestor" for A and B.
type aBase struct {
ABCD // embed the interface. As it is just a pointer, it has to be initialized!
}


// Bar is common to A and B.
func (a *aBase) Bar() {
a.Foo() // aBase has no method Foo defined, so it calls Foo method of embedded interface.
}


// a class, not exported
type a struct {
aBase
}


func (a *a) Foo() {
fmt.Println("A.Foo()")
}


// b class, not exported
type b struct {
aBase
}


func (b *b) Foo() {
fmt.Println("B.Foo()")
}


//////////////////////////////////////////////////////////////////////
// Now, public functions and methods.


// ABCD describes all exported methods of A and B.
type ABCD interface {
Foo()
Bar()
}


// NewA returns new struct a
func NewA() ABCD {
a := &a{}
a.ABCD = a
return a
}


// NewB returns new struct b
func NewB() ABCD {
b := &b{}
b.ABCD = b
return b
}

Try it on Go Playground: https://play.golang.org/p/0Zcs_arturP

小开

Coming from C++/Python, where OOP is much better represented, and discovering go (now everything is web or web related, right?!) I too stumbled upon this issue. I feel that OOP in go is only half-baked. With embedding (struct's anonymous fields), methods of the inner type come along for free, inducing the idea of inheritance, only to learn later on the limitations. However, playing a bit with embedded interfaces within structs and with a bit of discipline, C++-like constructors, inheritance, polymorphism, and methods override can be emulated.

Considering the example - https://play.golang.org/p/nqt0haPYt2x

package main


import (
"bytes"
"fmt"
"log"
"math"
"unsafe"
)


//Emulate C++ like polymorphism in go, through template method design pattern


//========================== Shape interface ==============================
//like C++ abstract classes
type Shape interface {
Area() float32      //Shape's area
Perimeter() float32 //Shape's perimeter
Name() string       //Shape's name (like rectangle, circle, square etc.)
}


//====================== PrintableShapeInfo =============================
type PrintableShapeInfo struct {
Shape             //like C++ inheritance, although go has no such a thing
preetyPrintPrefix string
}


//Init a new PrintableShapeInfo object. The method is distinct so that it can be called from other contexts as well
//
//Remark: emulates the C++ constructor init part
func (printableShapeInfo *PrintableShapeInfo) Init(preetyPrintPrefix string) {
printableShapeInfo.preetyPrintPrefix = preetyPrintPrefix
}


//The central method emulates the template method design pattern. It prints some info about a shape by dynamically calling (through pointers) the right methods
//
//Remark: the design patterns best practices recommend to favor composition over inheritance (i.e. model a ShapeInfoPrinter class, which takes a Shape interface and prints its info),
//for the sake of showcasting the template method pattern, the "go's inheritange" like model was chosen
func (printableShapeInfo *PrintableShapeInfo) PrintInfo() {
log.Println("PrintableShapeInfo::PrintInfo")
fmt.Printf("%s PrintableShapeInfo::PrintInfo - %s:\n",
printableShapeInfo.preetyPrintPrefix, printableShapeInfo.Name()) //dynamically calls (through a pointer) a shape's Name method (like Rectangle.Name or Circle.Name or Square.Name)
fmt.Printf("\tArea: %f\n", printableShapeInfo.Area())           //dynamically calls (through a pointer) a shape's Area method (like Rectangle.Area or Circle.Area or Square.Area)
fmt.Printf("\tPerimeter: %f\n", printableShapeInfo.Perimeter()) //dynamically calls (through a pointer) a shape's Perimeter method (like Rectangle.Perimeter or Circle.Perimeter or Square.Perimeter)
}


//====================== Rectangle =============================
type Rectangle struct {
PrintableShapeInfo         //like C++ inheritence, although go has no such a thing
width              float32 //rectangle's width
height             float32 //rectangle's heigh
}


//Creates and init a new rectangle object and properly set its Shape's interface methods set (similar to C++ class' vtable)
//
//Remark: emulates the C++ constructor
func NewRectangle(width float32, height float32) *Rectangle {
log.Println("NewRectangle")
rectangle := new(Rectangle)   //allocate data
rectangle.Shape = rectangle   //set the Shape's specific vtable with the Rectangle's methods. Critical for template method pattern
rectangle.Init(width, height) //init class
return rectangle
}


//Init a new rectangle object. The method is distinct so that it can be called from other contexts as well (such as a square Init method. See below)
//
//Remark: emulates the C++ constructor init part
func (rectangle *Rectangle) Init(width float32, height float32) {
log.Println("Rectangle::Init")
//call the base's PrintableShapeInfo struct Init method
rectangle.PrintableShapeInfo.Init("###")
rectangle.width = width
rectangle.height = height
}


//Compute the rectangle's area
func (rectangle *Rectangle) Area() float32 {
log.Println("Rectangle::Area")
return float32(rectangle.width * rectangle.height)
}


//Compute the rectangle's perimeter
func (rectangle *Rectangle) Perimeter() float32 {
log.Println("Rectangle::Perimeter")
return float32(2 * (rectangle.width + rectangle.height))
}


//Get the rectangle's object name
func (rectangle *Rectangle) Name() string {
log.Println("Rectangle::Name")
return "rectangle"
}


//====================== Circle =============================
type Circle struct {
PrintableShapeInfo         //like C++ inheritence, although go has no such a thing
radius             float32 //circle's radius
}


//Creates and init a new circle object and properly set its Shape's interface methods set (similar to C++ class' vtable)
//
//Remark: emulates the C++ constructor
func NewCircle(radius float32) *Circle {
log.Println("NewCircle")
circle := new(Circle) //allocate data
circle.Shape = circle //set the Shape's specific vtable with the Rectangle's methods. Critical for template method pattern
circle.Init(radius)   //init class
return circle
}


//Init a new circle object. The method is distinct so that it can be called from other contexts as well if needed
//
//Remark: emulates the C++ constructor init part
func (circle *Circle) Init(radius float32) {
log.Println("Circle::Init")
//call the base's PrintableShapeInfo struct Init method
circle.PrintableShapeInfo.Init("ooo")
circle.radius = radius
}


//Compute the circle's area
func (circle *Circle) Area() float32 {
log.Println("Circle::Area")
return math.Pi * float32(circle.radius*circle.radius)
}


//Compute the circle's perimeter
func (circle *Circle) Perimeter() float32 {
log.Println("Circle::Perimeter")
return 2 * math.Pi * float32(circle.radius)
}


//Get the circle's object name
func (circle *Circle) Name() string {
log.Println("Circle::Name")
return "circle"
}


//====================== Rectangle =============================
//Implement Square in terms of Rectangle
type Square struct {
Rectangle //like C++ inheritance, although go has no such a thing
}


//Creates and init a new square object and properly set its Shape's interface methods set (similar to C++ class' vtable)
//
//Remark: emulates the C++ constructor init
func NewSquare(width float32) *Square {
log.Println("NewSquare")
square := new(Square) //allocate data
square.Shape = square //set the Shape's specific vtable with the Rectangle's methods. Critical for template method pattern
square.Init(width)    //init class
return square
}


//Init a new square object. The method is distinct so that it can be called from other contexts as well if needed
//
//Remark: emulates the C++ constructor init part
func (square *Square) Init(width float32) {
log.Println("Square::Init")
//since the Rectangle field is anonymous it's nice that we can directly call its un-overwritten methods but we can still access it, as named Rectangle, along with its (even overwritten) methods
square.Rectangle.Init(width, width) //call Rectangle's init to initialize its members. Since Square is implemented in Rectangle's terms, there nothing else needed
}


//Compute the square's area
func (square *Square) Area() float32 {
log.Println("Square::Area")
//since the Rectangle field is anonymous it's nice that we can directly call it's un-overwritten methods but we can still access it, as named Rectangle, along with it's (even overwritten) methods
return square.Rectangle.Area()
}


//Compute the square's perimeter
func (square *Square) Perimeter() float32 {
log.Println("Square::Perimeter")
//since the Rectangle field is anonymous it's nice that we can directly call it's un-overwritten methods but we can still access it, as named Rectangle, along with it's (even overwritten) methods
return square.Rectangle.Perimeter()
}


//Get the square's object name
func (square *Square) Name() string {
log.Println("Square::Name")
return "square"
}


func main() {
//initialize log subsystem so that we can display them at the main's end
// bufWriter := bytes.NewBuffer()
logStringWriter := bytes.NewBufferString("")
log.SetOutput(logStringWriter)


rectangle := NewRectangle(2, 3) //create a Rectangle object
rectangle.PrintInfo()           //should manifest polymorphism behavior by calling Rectangle's Area, Perimeter and Name methods


circle := NewCircle(2) //create a Circle object
circle.PrintInfo()     //should manifest polymorphism behavior by calling Circle's Area, Perimeter and Name methods


square := NewSquare(3) //create a Square object
square.PrintInfo()     //should manifest polymorphism behavior by calling Square's Area, Perimeter and Name methods


//print constructs sizes
fmt.Printf(`
Go constructs sizes:
Shape interface size as seen by Rectangle struct:  %d
`, unsafe.Sizeof(rectangle.Shape))
fmt.Printf("\tRectangle struct size: %d", unsafe.Sizeof(rectangle))


fmt.Printf(`
Shape interface size as seen by Circle struct:  %d
`, unsafe.Sizeof(circle.Shape))
fmt.Printf("\tCircle struct size: %d", unsafe.Sizeof(circle))


fmt.Printf(`
Shape interface size as seen by Square struct:  %d
`, unsafe.Sizeof(square.Shape))
fmt.Printf("\tCircle struct size: %d", unsafe.Sizeof(square))


//print the logs
fmt.Println("\n\nDumping traces")
fmt.Print(logStringWriter)
return
}

The central method (template method) is PrintInfo which, called for any defined shape, works as expected, by calling the right Area, Perimeter, and Name methods. Ex. circle.PrintInfo() will call circle.Area, circle.Perimeter and circle.Name.

The constructor functions, NewRectangle, NewCircle, and NewSquare construct shape objects and they're split into three steps:

space allocation
methods set (C++ like vtable) init, needed for polymorphic behavior
struct members initialization, through Init methods

The struct member initialization is a distinct step for better code reuse. For example, Rectangle Init calls the base PrintableShapeInfo Init method while the Square Init method calls the base Rectangle Init (which calls PrintableShapeInfo Init, as said before).

Also, due to interfaces embedding, the object sizes increases only a bit, with a pair of pointers to the methods set and data area as can be seen in the example output.

I think the code looks pretty decent and the only concern being if specifically setting the Shape's interface method set (as the case of NewRectangle, NewCircle, and NewSquare functions) would trigger some side effects, as the code appears to work correctly?!