I am a new to golang. I need to design a function to create object of differing types based on input. But I failed to figure out how to design the interface. Here comes my code:
package main
import (
"fmt"
)
type AA struct{
name string
}
func (this *AA) say(){
fmt.Println("==========>AA")
}
type BB struct{
*AA
age int
}
func (this *BB) say(){
fmt.Println("==========>BB")
}
func ObjectFactory(type int) *AA {
if type ==1 {
return new(AA)
}else{
return new(BB)
}
}
func main() {
obj1 := ObjectFactory(0)
obj1.say()
obj2 := ObjectFactory(0)
obj2.say()
}
The compiler tells me error no matter I ask ObjectFactory return *AA or interface{}. How can I make it work?
First off, using type as a variable name is disallowed in go (see the spec). That is your first problem.
The return type of object factory is *AA. This means that it can only return variables of type *AA, which causes the return of type of BB to fail. As defined in the spec, go doesn't have type inheritance, just struct embedding.
If you create an interface called sayer, you can use that instead of *AA in your ObjectFactory function.
type sayer interface {
say()
}
You probably want to use this interface when trying to get multiple dispatch (as demonstrated in the code below (see on play.golang.org as well).
Try this code:
package main
import (
"fmt"
)
type sayer interface {
say()
}
type AA struct{
name string
}
func (this *AA) say(){
fmt.Println("==========>AA")
}
type BB struct{
*AA
age int
}
func (this *BB) say(){
fmt.Println("==========>BB")
}
func ObjectFactory(typeNum int) sayer {
if typeNum ==1 {
return new(AA)
}else{
return new(BB)
}
}
func main() {
obj1 := ObjectFactory(1)
obj1.say()
obj2 := ObjectFactory(0)
obj2.say()
}
Related
I have a basic handler, that uses an 3rd party package messager.
The package returns a concrete struct of messager.Shape provided by a Get method.
messager.go
type Shape struct {
Value struct {
Type string
Other int
Active bool
}
Markers struct {
Range int
Name string
}
ID string
}
...
func (m *Messager) Get(ID string) (Shape, error) {
...
}
handler.go
package handler
import (
"fmt"
"github.com/fake/message"
)
type Getter interface {
Get(ID string) (messager.Shape, error)
}
type Handler struct {
messager Getter
}
func NewHandler(g Getter) *Handler {
return &Handler{ messager: g }
}
func (h *Handler) DoSomething() {
fmt.Println(h.messager.Get("some-id").Value.Type)
}
The handler only really cares that something can provide a struct with a matching nested Value.Type string.
I attempted defining a struct for the interface that wanted just:
type ValueType struct {
Value struct {
Type string
}
}
type Getter interface {
Get(ID string) (ValueType, error)
}
but the compiler rejects it when creating a NewHandler as the messager.Get method type does not match the interface. What am I suppose to do here? Is it appropriate for my Handler to reference the package struct? Should the package not be returning something so definitive?
I appreciate the example is a bit contrived but just trying to work out what's correct.
I'm confused about how to check if an attribute exists in an object's dynamic struct. I.e. if we have the following structs:
type Animal struct {
Name string
Origin string
}
type Bird struct {
Animal
Speed float32
CanFly bool
}
type Bear struct {
Animal
Lazy bool
}
And now I have a function using Animal as a parameter:
func checkAminalSpeed (a Animal){
// if the struct of current animal doesn't have the Speed attribute
// print ("I don't have a speed")
//otherwise, return the speed of this animal
}
This function is trying to check the varible's runtime type to choose action.
I'd like to know in this case, how to write this checkAminalSpeed function? Thanks!
Go does not support inheritance, but perhaps you'll find the following approach tolerable.
Use an interface to define the Animal's behaviour:
type Animal interface {
GetName() string
GetOrigin() string
GetSpeed() float32
}
Use a "base" type that will contain common fields and also implement the behaviour:
type AnimalBase struct {
Name string
Origin string
}
func (a AnimalBase) GetName() string { return a.Name }
func (a AnimalBase) GetOrigin() string { return a.Origin }
func (a AnimalBase) GetSpeed() float32 { return -1 }
Embed the "base" type and override any behaviour you need to:
type Bird struct {
AnimalBase
Speed float32
CanFly bool
}
func (b Bird) GetSpeed() float32 { return b.Speed }
And then...
func checkAminalSpeed(a Animal) {
if speed := a.GetSpeed(); speed == -1 {
fmt.Println("I don't have speed")
} else {
fmt.Println(speed)
}
}
https://play.golang.org/p/KIjfC7Rdyls
mkopriva is right. Go does not support inheritance, you can use reflect and interface{} also
ps: reflect cost more time than interface
package main
import (
"fmt"
"reflect"
)
type Animal struct {
Name string
Origin string
}
type Bird struct {
Animal
Speed float32
CanFly bool
}
type Bear struct {
Animal
Lazy bool
}
func checkAminalSpeed (a interface{}){
v := reflect.ValueOf(a)
if f, ok := v.Type().FieldByName("Speed"); ok{
fmt.Printf("%v\n", f)
}
}
func main() {
checkAminalSpeed(Bird{})
checkAminalSpeed(Bear{})
}
Foreign application API gives me a list of names in JSON format. I need modify all of those.
But I do not like to write some loop for it (especially after Python using with reflection and stuff)
Is there any method to write something like this in Go?
type MyIncredibleType struct {
Name ModifyName // ModifyName is not a type!
}
func ModifyName(input string) string {
return input + ".com"
}
The expected behavior of this is:
a := MyIncredibleType{Name: "Abracadabra"}
print(a.Name) // Abracadabra.com
This seems pretty straight forward to me, assuming I understand your question correctly:
// ModifyName func
func ModifyName(input string) string {
return fmt.Sprintf("%v.com", input)
}
If you wish to achieve this within the type itself, without modifying (mutating) the internal state:
type MyType sturct {
name string // unexported
}
// accessor func to return name
func (t MyType) Name() string {
return t.name
}
// accessor func to return modified name
func (t MyType) ModifiedName() string {
return fmt.Sprintf("%v.com", t.name)
}
If you want to modify the internal state:
type MyType struct {
name string
}
// mutator func (note the pointer for pass by reference)
func (t *MyType) ModifyName(input string) {
t.name = fmt.Sprintf("%v.com", input)
}
// accessor (note no pointer for pass by value)
func (t MyType) Name() string {
return t.name
}
This is is not possible in GO. That's not how struct works in Go.
type MyIncredibleType struct {
Name ModifyName `json:"name"` // ModifyName is not a type!
}
you can only define Built-in types for your fields of struct or you can define Composite Literal types.
Composite literals construct values for structs, arrays, slices, and
maps and create a new value each time they are evaluated. They consist
of the type of the literal followed by a brace-bound list of elements.
Each element may optionally be preceded by a corresponding key.
Try to create a method receiver of struct which you are using to parse json coming from the api to modify the name. That will let you achieve something similar to what you want.
package main
import (
"fmt"
)
type MyIncredibleType struct {
Name string `json:"name"` // ModifyName is not a type!
}
func(myIncredibleType *MyIncredibleType) ModifyName() string {
return myIncredibleType.Name+".com"
}
func main() {
a := MyIncredibleType{Name: "Abracadabra"}
name := a.ModifyName()
fmt.Printf("%s",name)
}
Playground Example
Or you can pass an interface which will wrap any struct value with name field and then use Type assertion to get the underlying value to modify the same and return the result:
package main
import (
"fmt"
)
type MyIncredibleType struct {
Name string `json:"name"` // ModifyName is not a type!
}
func ModifyName(input interface{}) string{
return input.(interface{}).(string)+".com"
}
func main() {
a := MyIncredibleType{Name: "Abracadabra"}
name := ModifyName(a.Name)
fmt.Printf("%s",name)
}
Working code on Go Playground
For more information also go through Golang method Declarations on how to create receivers.
Suppose I have code, where a function accepts another one as an argument:
type Person struct {
Name string
}
func personBuilder() * Person {
return &Person{Name: "John"}
}
func printRetrievedItem(callback func() interface {}){
fmt.Print(callback());
}
func doStuff(){
printRetrievedItem(personBuilder);
}
This results in error cannot use personBuilder (type func() *Person) as type func() interface {} in function argument. If I change personBuilder return type to interface{}, it works, but in real project I'm working on I want to have a concrete type for clear design and TDD purposes.
Does Go support such method signature generalization? What are the workarounds, if you could not change the personBuilder part (e.g. you have a lot parameterless functions that return different type of struct, and you want to build a consumer function that accepts any of those builders as argument)?
One workaround is to define an inline function that calls personBuilder.
printRetrievedItem(func() interface{} {return personBuilder()});
Playground
You can create an interface with a method that returns an interface{}:
type Thinger interface {
Thing() interface{}
}
func (p *Person) Thing() interface{} {
return p
}
func printRetrievedItem(t Thinger){
fmt.Print(t.Thing());
}
func doStuff(){
printRetrievedItem(personBuilder);
}
This is just an example, please use a better name!
To answer your question, fun() A is not a func() interface{}, for the same reason that []A is not an []interface{}. It's explained very well in the go wiki.
Either do a wrapper like #GrzegorzŻur suggested or define your own interface and make your xxxxBuilder return it:
type Namer interface {
Name() string
}
type Person struct {
name string
}
func (p *Person) Name() string {
return p.name
}
func personBuilder() Namer {
return &Person{name: "John"}
}
func printRetrievedItem(callback func() Namer) {
fmt.Printf("%T: %v", callback(), callback().Name())
}
You can use pkg reflect for this. (Note however that the solution of #OneOfOne is more idiomatic).
package main
import (
"fmt"
"reflect"
)
type Person struct {
Name string
}
func personBuilder() *Person {
return &Person{Name: "John"}
}
func printRetrievedItem(callback interface{}) {
vals := reflect.ValueOf(callback).Call([]reflect.Value{})
fmt.Println(vals[0].Interface())
}
func main() {
printRetrievedItem(personBuilder) // &{John}
printRetrievedItem(func() string { return "hello" }) // hello
}
Here's an example in the playground.
In the following test code I would like to have both mytype and the doPrivate method private, so that only members of mytype can access it, but not other types\functions in the scope of the mypackage package.
Can I do this in golang?
package mypackage
type mytype struct {
size string
hash uint32
}
func (r *mytype) doPrivate() string {
return r.size
}
func (r *mytype) Do() string {
return doPrivate("dsdsd")
}
Fields size and hash as well as the doPrivate method should be encapsulated and no other type should have access to them.
In Go, an identifier that starts with a capital letter is exported from the package, and can be accessed by anyone outside the package that declares it.
If an identifier starts with a lower case letter, it can only be accessed from within the package.
If you need members in a type to only be accessed by members of that type, you then need to place that type and its member functions in a separate package, as the only type in that package.
That's not how "privacy" works in Go: the granularity of privacy is the package.
If you really want only the members of mytype to access some fields, then you must isolate the struct and the functions in their own package.
But that's not the usual practice. Whether Go is OOP or not is debatable but clearly the practice isn't to encapsulate the code by a struct like you seem to want to do. Usually a package is small enough to be coherent: if you don't want to access fields from within the package, don't access them.
You can create an interface with the method you wish to expose and only access the object when wrapped into that interface.
package main
type mytype struct {
size string
hash uint32
}
// interface for exposed methods
type myinterface interface {
do() string
}
// constructor (optional)
func newMytype(size string, hash uint32) myinterface {
return &mytype{size, hash}
}
func (r *mytype) doPrivate() string {
return r.size
}
func (r *mytype) do() string {
return r.doPrivate()
}
func main() {
// with constructor
t := newMytype("100", 100)
t.do()
// t.doPrivate() // t.doPrivate undefined (type myinterface has no field or method doPrivate)
// without constructor
t2:= myinterface(&mytype{"100", 100})
t2.do()
// t.doPrivate() // t.doPrivate undefined (type myinterface has no field or method doPrivate)doPrivate)
}
https://play.golang.org/p/awjIIj8Kwms
You cannot do this in Go. Visibility is on a per package level only. But you may split your package into two.
In one module there can be any number of packages.
Public/Private works only across one package.
All public fields, methods and functions starts with uppercase char.
All private fields, methods and functions starts with lowercase char.
To add package to your module or program just create a lowercase folder and add package name to all files inside. Here is the example.
./main.go
./foo/foo.go
./foo/MyStruct.go
file ./foo/foo.go:
package foo
import "fmt"
func SomePublicFuncInFoo() {
somePrivateFuncInFoo()
}
func somePrivateFuncInFoo() {
fmt.Println("somePrivateFuncInFoo call")
}
file ./foo/MyStruct.go:
package foo
import "fmt"
type MyStruct struct {
MyPublicField string // starts with uppercase char
myPrivateField string // starts with lowercase char
}
func NewMyStruct(publicField string, privateField string) *MyStruct {
return &MyStruct{
MyPublicField: publicField,
myPrivateField: privateField,
}
}
func (self *MyStruct) SomePublicMethod() {
self.privateMethod()
}
func (self *MyStruct) privateMethod() {
fmt.Println("MyStruct", self.MyPublicField, self.myPrivateField)
}
file ./main.go:
package main
import (
"fmt"
"{your-module-name}/foo" // this line should be added by your IDE
)
func main() {
foo.SomePublicFuncInFoo()
myStruct := foo.NewMyStruct("string1", "string2")
fmt.Println("myStruct.MyPublicField=", myStruct.MyPublicField)
myStruct.SomePublicMethod()
}
You can have private variables and functions in Go, but the trick is that you simply don't define them in the struct. Bind them to the call stack of a closure, and simply don't return them.
package main
import (
"fmt"
)
type mytype struct {
Do func() string
}
func MyType(size string, hash uint32) mytype {
doPrivate := func() string {
return size
}
return mytype{
Do: func() string {
return doPrivate()
},
}
}
func main() {
instance := MyType("100", 100)
fmt.Println(instance.Do())
}