I have two different interfaces (from two different packages) that I want to implement. But they conflict, like this:
type InterfaceA interface {
Init()
}
type InterfaceB interface {
Init(name string)
}
type Implementer struct {} // Wants to implement A and B
func (i Implementer) Init() {}
func (i Implementer) Init(name string) {} // Compiler complains
It says "Method redeclared". How can one struct implement both interfaces?
As already answered, this is not possible since Golang does not (and probably will not) support method overloading.
Look at Golang FAQ:
Experience with other languages told us that having a variety of methods with the same name but different signatures was occasionally useful but that it could also be confusing and fragile in practice. Matching only by name and requiring consistency in the types was a major simplifying decision in Go's type system.
It is not possible.
In go you must have a single method signature.
You should rename one method.
The method signatures must match. If you want dependency injection I would recommend the functional option pattern. Functional options are functions that return other functions that are called in a loop in the constructor. Here is an example of how to use functional options and the basics of interfaces in go.
package main
import (
"fmt"
"strconv"
)
type SomeData struct {
data string
}
// SomeData and SomeOtherData both implement SomeInterface and SomeOtherInterface
// SomeInterface and SomeOtherInterface both implement each other.
type SomeInterface interface {
String() string
Set(data string)
}
func (s *SomeData)String() string {
return s.data
}
func (s *SomeData)Set(data string) {
s.data = data
}
// SetDataOption is a functional option that can be used to inject a constructor dep
func SetDataOption(data string) func(*SomeData) {
return func(s *SomeData) {
s.Set(data)
}
}
// NewSomeData is the constructor; it takes in 0 to many functional options and calls each one in a loop.
func NewSomeData(options ...func(s *SomeData)) SomeInterface {
s := new(SomeData)
for _, o := range options {
o(s)
}
return s
}
//********************
type SomeOtherData struct {
data string
i int
}
type SomeOtherInterface interface {
String() string
Set(data string)
}
func (s *SomeOtherData)String() string {
return s.data + " " + strconv.Itoa(s.i)
}
func (s *SomeOtherData)Set(data string) {
s.data = data
}
func SetOtherDataOption(data string) func(*SomeOtherData) {
return func(s *SomeOtherData) {
s.Set(data)
}
}
func SetOtherIntOption(i int) func(*SomeOtherData) {
return func(s *SomeOtherData) {
s.i = i
}
}
// NewSomeOther data works just like NewSomeData only in this case, there are more options to choose from
// you can use none or any of them.
func NewSomeOtherData(options ...func(s *SomeOtherData)) SomeOtherInterface {
s := new(SomeOtherData)
for _, o := range options {
o(s)
}
return s
}
//*********************************
// HandleData accepts an interface
// Regardless of which underlying struct is in the interface, this function will handle
// either by calling the methods on the underlying struct.
func HandleData(si SomeInterface) {
fmt.Println(si) // fmt.Println calls the String() method of your struct if it has one using the Stringer interface
}
func main() {
someData := NewSomeData(SetDataOption("Optional constructor dep"))
someOtherData := NewSomeOtherData(SetOtherDataOption("Other optional constructor dep"), SetOtherIntOption(42))
HandleData(someData) // calls SomeData.String()
HandleData(someOtherData) // calls SomeOtherData.String()
someOtherData = someData // assign the first interface to the second, this works because they both implement each other.
HandleData(someOtherData) // calls SomeData.String() because there is a SomeData in the someOtherData variable.
}
I have the following
https://go.dev/play/p/s3VizCW1pio
func main() {
type Book struct {
// Bad: pointer changes zero value usefulness
io.ReadWriter
}
// later
var b Book
b.Read([]byte{'1'}) // panic nil value
}
From https://github.com/uber-go/guide
My question is, when we add io.ReadWriter to Book, should it compile at all if we haven't implemented the interface io.ReadWriter for Book?
There's clearly a gap in my understanding and any help would be greatly appreciated
Embedded Types
What you have here is called an embedded type. Embedded types allow you to place other types at the root of a struct and refer to them using their unqualified type name. The methods of the embedded type are available from the parent struct. From a compilation failure perspective, there is no difference between what you have and this:
type Book struct {
writer io.ReadWriter
}
In other words, your example does not require that you specify a value for writer for the same reason that my example does not -- an embedded value is treated just like any other value other than the embedded type's methods are available from the parent type. In either case, if a value is not provided for the field, then you'll get a runtime nil pointer error.
Should it compile at all if we haven't implemented the interface io.ReadWriter for Book?
It will compile but panic if you try to access the field or its methods. The solution is to provide an implementation by assigning to the embedded field's name.
package main
import (
"bytes"
"io"
)
type Book struct {
io.ReadWriter
}
func main() {
book := Book{
ReadWriter: &bytes.Buffer{},
}
}
Implementing the Interface
If all you want to do is create a Book struct that implements the io.ReadWriter interface, then you can get rid of the io.ReadWriter field on the struct entirely and just do something like this:
package main
import (
"io"
)
var _ io.ReadWriter = &Book{}
type Book struct {}
func (b *Book) Read(p []byte) (n int, err error) {
// Custom implementation here
return 0, err
}
func (b *Book) Write(p []byte) (n int, err error) {
// Custom implementation here
return 0, err
}
func main() {
book := Book{}
}
Custom Wrappers
You can do some pretty cool stuff by embedding types such as wrapping a lower-level implementation of io.ReadWriter (in this example) with your own custom logic before calling the embedded implementation.
var _ io.ReadWriter = &Book{}
type Book struct {
io.ReadWriter
}
func (b *Book) Read(p []byte) (n int, err error) {
// add a custom behavior before calling the underlying read method
return b.ReadWriter.Read(p)
}
func (b *Book) Write(p []byte) (n int, err error) {
// add a custom behavior before calling the underlying write method
return b.ReadWriter.Write(p)
}
func main() {
book := Book{
ReadWriter: &bytes.Buffer{},
}
}
What you did with type Book is that you embedded an io.ReadWriter into it. That means, Book type has a field of type io.ReadWriter, and since it is embedded, Book now has all the methods defined for io.ReadWriter. You have to set that field to point to an implementation of io.ReadWriter to use it:
var b Book
myFile,err:=op.Open("someFile") // myFile implements io.ReadWriter
b.ReadWriter=myFile
data:=make([]byte,1)
b.Read(data) // will read from myFile
I am using pagerduty go sdk to do a bunch of api requests.
Particularly I am making use of
func NewClient(authToken string) *Client
to create a new Client type. I want to add some utility functions to my own work to *Client. I tried doing this:
type BetterPdClient *pagerduty.Client
func NewClient(auth string) BetterPdClient {
return pagerduty.NewClient(auth)
}
func (b *BetterPdClient) DoSomething() {
b.GetIncident(....)
}
func main() {
pd_client := NewClient("")
fmt.Println(pd_client)
pd_client.DoSomething()
}
But I get the following error:
invalid receiver type *BetterPdClient (BetterPdClient is a pointer type)
I understand that DoSomething() is expecting a pointer as caller. Only other way I could think of is sending the ptr as a function argument:
func NewClient(auth string) *pagerduty.Client {
return pagerduty.NewClient(auth)
}
func DoSomething(cl *pagerduty.Client) {
fmt.Println(cl)
}
func main() {
pd_client := NewClient("")
fmt.Println(pd_client)
DoSomething(pd_client)
}
Is there a better way?
Declaring a type as a pointer to another type is almost never what you want because Go doesn't allow you to add methods to that new type, nor to the pointer of that type as you've already figured out yourself. This one doesn't compile either:
type T struct{}
type P *T
func (P) M() {}
If you want to "extend" a type without "hiding" it's existing functionality your best bet is to embed it in a struct.
type T struct{
// ...
}
func (T) M() {}
type U struct {
*T
}
func NewU() *U {
return &U{&T{}}
}
func (U) N() {}
func main() {
u := NewU()
u.M()
u.N()
}
And what I mean by "hiding existing functionality" is that when you define a new type in terms of another, already existing type, your new type will not get direct access to the methods of the existing type. All you're doing is just saying that your new type should have the same structure as the already existing type. Although it's worth pointing out that this property gives you the ability to convert one type to the other...
type T struct{
// ...
}
func (T) M() {}
type U T
func NewU() *U {
return &U{}
}
func (U) N() {}
func main() {
u := NewU()
u.M() // compile error
u.N()
// convert *U to *T and then call M
(*T)(u).M()
}
Currently, I'm having two files in two packages.
File somepkg/something.go
package somepkg
// ----------------------------------------------------------------------------
// Interfaces
// ----------------------------------------------------------------------------
type SomeThing interface {
// some other methods
Handle(handler *Handler)
}
type SomeThingImpl struct {
handlers []Handler
}
type Handler interface {
IncomingCall(request *IncomingRequest)
}
// ----------------------------------------------------------------------------
// SomeThingImpl implementation
// ----------------------------------------------------------------------------
func NewSomeThing() SomeThing {
u := new(SomethingImpl)
// some more operations with u
return u
}
func (l *SomeThingImpl) Handle(handler *Handler) {
fmt.Printf("handler: %s", handler)
}
File main.go
package main
import (
"fmt"
)
type MyHandler struct {}
func (h *MyHandler) IncomingCall(request *somepkg.IncomingRequest) {
fmt.Printf("Handler!")
}
func main() {
mySomeThing := somepkg.NewSomeThing()
handler := new(MyHandler)
// works so far.
mySomeThing.Handle(handler) // <-- here the compilation error occurs
}
trying to run go build yields the following compilation error:
{...}\main.go:20: cannot use handler (type *MyHandler) as type *somepkg.Handler in argument to mySomething.Handle:
*somepkg.Handler is pointer to interface, not interface
whereas main.go:20 refers to the line above where I'm calling mySomeThing.Handle(handler).
Actually, both MyHandler and somepkg.Handler seem to be pointers. Both of them implement the same methods.
Why does the compiler not consider these types to be compatible ?
You have this method;
func (l *SomeThingImpl) Handle(handler *Handler) {
fmt.Printf("handler: %s", handler)
}
Defined to take an interface pointer which is likely not what you want. What you're actually looking for is to have *MyHandler implement the Handler interface rather than MyHandler (or both could) so that you can pass your reference type into the method.
//new method sig
Handle(handler Handler)
// my preferred syntax for assignment here
handler := &MyHandler{}
// works just fine
mySomeThing.Handle(handler)
If you really want to keep the method sig how you have it, to make your code work you just need to do mySomeThing.Handle(&handler) but I'm doubtful that's actually what you want to do.
btw; In your code *MyHandler is already implementing the interface due to this method func (h *MyHandler) IncomingCall(request *somepkg.IncomingRequest) haveing *MyHandler as the receiving type as apposed to func (h MyHandler) which would make it so only MyHandler implements the Handler interface. Some little Go nuance with recievers and interfaces there.
Never use a pointer to an interface (well, unless you know exactly why you would want to do so)
In order to pass that into the defined function, you would need to create the *somepkg.Handler explicitly (but don't really do this)
var handler somepkg.Handler = new(MyHandler)
mySomeThing.Handle(&handler)
If you define Handle as
Handle(handler Handler)
any type that satisfies the somepkg.Handler interface can be passed in directly.
sort package:
type Interface interface {
Len() int
Less(i, j int) bool
Swap(i, j int)
}
...
type reverse struct {
Interface
}
What is the meaning of anonymous interface Interface in struct reverse?
In this way reverse implements the sort.Interface and we can override a specific method
without having to define all the others
type reverse struct {
// This embedded Interface permits Reverse to use the methods of
// another Interface implementation.
Interface
}
Notice how here it swaps (j,i) instead of (i,j) and also this is the only method declared for the struct reverse even if reverse implement sort.Interface
// Less returns the opposite of the embedded implementation's Less method.
func (r reverse) Less(i, j int) bool {
return r.Interface.Less(j, i)
}
Whatever struct is passed inside this method we convert it to a new reverse struct.
// Reverse returns the reverse order for data.
func Reverse(data Interface) Interface {
return &reverse{data}
}
The real value comes if you think what would you have to do if this approach was not possible.
Add another Reverse method to the sort.Interface ?
Create another ReverseInterface ?
... ?
Any of this change would require many many more lines of code across thousands of packages that want to use the standard reverse functionality.
Ok, the accepted answer helped me understand, but I decided to post an explanation which I think suits better my way of thinking.
The "Effective Go" has example of interfaces having embedded other interfaces:
// ReadWriter is the interface that combines the Reader and Writer interfaces.
type ReadWriter interface {
Reader
Writer
}
and a struct having embedded other structs:
// ReadWriter stores pointers to a Reader and a Writer.
// It implements io.ReadWriter.
type ReadWriter struct {
*Reader // *bufio.Reader
*Writer // *bufio.Writer
}
But there is no mention of a struct having embedded an interface. I was confused seeing this in sort package:
type Interface interface {
Len() int
Less(i, j int) bool
Swap(i, j int)
}
...
type reverse struct {
Interface
}
But the idea is simple. It's almost the same as:
type reverse struct {
IntSlice // IntSlice struct attaches the methods of Interface to []int, sorting in increasing order
}
methods of IntSlice being promoted to reverse.
And this:
type reverse struct {
Interface
}
means that sort.reverse can embed any struct that implements interface sort.Interface and whatever methods that interface has, they will be promoted to reverse.
sort.Interface has method Less(i, j int) bool which now can be overridden:
// Less returns the opposite of the embedded implementation's Less method.
func (r reverse) Less(i, j int) bool {
return r.Interface.Less(j, i)
}
My confusion in understanding
type reverse struct {
Interface
}
was that I thought that a struct always has fixed structure, i.e. fixed number of fields of fixed types.
But the following proves me wrong:
package main
import "fmt"
// some interface
type Stringer interface {
String() string
}
// a struct that implements Stringer interface
type Struct1 struct {
field1 string
}
func (s Struct1) String() string {
return s.field1
}
// another struct that implements Stringer interface, but has a different set of fields
type Struct2 struct {
field1 []string
dummy bool
}
func (s Struct2) String() string {
return fmt.Sprintf("%v, %v", s.field1, s.dummy)
}
// container that can embedd any struct which implements Stringer interface
type StringerContainer struct {
Stringer
}
func main() {
// the following prints: This is Struct1
fmt.Println(StringerContainer{Struct1{"This is Struct1"}})
// the following prints: [This is Struct1], true
fmt.Println(StringerContainer{Struct2{[]string{"This", "is", "Struct1"}, true}})
// the following does not compile:
// cannot use "This is a type that does not implement Stringer" (type string)
// as type Stringer in field value:
// string does not implement Stringer (missing String method)
fmt.Println(StringerContainer{"This is a type that does not implement Stringer"})
}
The statement
type reverse struct {
Interface
}
enables you to initialize reverse with everything that implements the interface Interface. Example:
&reverse{sort.Intslice([]int{1,2,3})}
This way, all methods implemented by the embedded Interface value get populated to the outside while you are still able to override some of them in reverse, for example Less to reverse the sorting.
This is what actually happens when you use sort.Reverse. You can read about embedding in the struct section of the spec.
I will give my explanation too. The sort package defines an unexported type reverse, which is a struct, that embeds Interface.
type reverse struct {
// This embedded Interface permits Reverse to use the methods of
// another Interface implementation.
Interface
}
This permits Reverse to use the methods of another Interface implementation. This is the so called composition, which is a powerful feature of Go.
The Less method for reverse calls the Less method of the embedded Interface value, but with the indices flipped, reversing the order of the sort results.
// Less returns the opposite of the embedded implementation's Less method.
func (r reverse) Less(i, j int) bool {
return r.Interface.Less(j, i)
}
Len and Swap the other two methods of reverse, are implicitly provided by the original Interface value because it is an embedded field. The exported Reverse function returns an instance of the reverse type that contains the original Interface value.
// Reverse returns the reverse order for data.
func Reverse(data Interface) Interface {
return &reverse{data}
}
I find this feature very helpful when writing mocks in tests.
Here is such an example:
package main_test
import (
"fmt"
"testing"
)
// Item represents the entity retrieved from the store
// It's not relevant in this example
type Item struct {
First, Last string
}
// Store abstracts the DB store
type Store interface {
Create(string, string) (*Item, error)
GetByID(string) (*Item, error)
Update(*Item) error
HealthCheck() error
Close() error
}
// this is a mock implementing Store interface
type storeMock struct {
Store
// healthy is false by default
healthy bool
}
// HealthCheck is mocked function
func (s *storeMock) HealthCheck() error {
if !s.healthy {
return fmt.Errorf("mock error")
}
return nil
}
// IsHealthy is the tested function
func IsHealthy(s Store) bool {
return s.HealthCheck() == nil
}
func TestIsHealthy(t *testing.T) {
mock := &storeMock{}
if IsHealthy(mock) {
t.Errorf("IsHealthy should return false")
}
mock = &storeMock{healthy: true}
if !IsHealthy(mock) {
t.Errorf("IsHealthy should return true")
}
}
By using:
type storeMock struct {
Store
...
}
One doesn't need to mock all Store methods. Only HealthCheck can be mocked, since only this method is used in the TestIsHealthy test.
Below the result of the test command:
$ go test -run '^TestIsHealthy$' ./main_test.go
ok command-line-arguments 0.003s
A real world example of this use case one can find when testing the AWS SDK.
To make it even more obvious, here is the ugly alternative - the minimum one needs to implement to satisfy the Store interface:
type storeMock struct {
healthy bool
}
func (s *storeMock) Create(a, b string) (i *Item, err error) {
return
}
func (s *storeMock) GetByID(a string) (i *Item, err error) {
return
}
func (s *storeMock) Update(i *Item) (err error) {
return
}
// HealthCheck is mocked function
func (s *storeMock) HealthCheck() error {
if !s.healthy {
return fmt.Errorf("mock error")
}
return nil
}
func (s *storeMock) Close() (err error) {
return
}
Embedding interfaces in a struct allows for partially "overriding" methods from the embedded interfaces. This, in turn, allows for delegation from the embedding struct to the embedded interface implementation.
The following example is taken from this blog post.
Suppose we want to have a socket connection with some additional functionality, like counting the total number of bytes read from it. We can define the following struct:
type StatsConn struct {
net.Conn
BytesRead uint64
}
StatsConn now implements the net.Conn interface and can be used anywhere a net.Conn is expected. When a StatsConn is initialized with a proper value implementing net.Conn for the embedded field, it "inherits" all the methods of that value; the key insight is, though, that we can intercept any method we wish, leaving all the others intact. For our purpose in this example, we'd like to intercept the Read method and record the number of bytes read:
func (sc *StatsConn) Read(p []byte) (int, error) {
n, err := sc.Conn.Read(p)
sc.BytesRead += uint64(n)
return n, err
}
To users of StatsConn, this change is transparent; we can still call Read on it and it will do what we expect (due to delegating to sc.Conn.Read), but it will also do additional bookkeeping.
It's critical to initialize a StatsConn properly, otherwise the field retains its default value nil causing a runtime error: invalid memory address or nil pointer dereference; for example:
conn, err := net.Dial("tcp", u.Host+":80")
if err != nil {
log.Fatal(err)
}
sconn := &StatsConn{conn, 0}
Here net.Dial returns a value that implements net.Conn, so we can use that to initialize the embedded field of StatsConn.
We can now pass our sconn to any function that expects a net.Conn argument, e.g:
resp, err := ioutil.ReadAll(sconn)
if err != nil {
log.Fatal(err)
And later we can access its BytesRead field to get the total.
This is an example of wrapping an interface. We created a new type that implements an existing interface, but reused an embedded value to implement most of the functionality. We could implement this without embedding by having an explicit conn field like this:
type StatsConn struct {
conn net.Conn
BytesRead uint64
}
And then writing forwarding methods for each method in the net.Conn interface, e.g.:
func (sc *StatsConn) Close() error {
return sc.conn.Close()
}
However, the net.Conn interface has many methods. Writing forwarding methods for all of them is tedious and unnecessary. Embedding the interface gives us all these forwarding methods for free, and we can override just the ones we need.
I will try another, low level approach to this.
Given the reverse struct:
type reverse struct {
Interface
}
This beside others means, that reverse struct has a field reverse.Interface, and as a struct fields, it can be nil or have value of type Interface.
If it is not nil, then the fields from the Interface are promoted to the "root" = reverse struct. It might be eclipsed by fields defined directly on the reverse struct, but that is not our case.
When You do something like:
foo := reverse{}, you can println it via fmt.Printf("%+v", foo) and got
{Interface:<nil>}
When you do the
foo := reverse{someInterfaceInstance}
It is equivalent of:
foo := reverse{Interface: someInterfaceInstance}
It feels to me like You declare expectation, that implementation of Interface API should by injected into your struct reverse in runtime. And this api will be then promoted to the root of struct reverse.
At the same time, this still allow inconsistency, where You have reverse struct instance with reverse.Interface = < Nil>, You compile it and get the panic on runtime.
When we look back to the specifically example of the reverse in OP, I can see it as a pattern, how you can replace/extend behaviour of some instance / implementation kind of in runtime contrary to working with types more like in compile time when You do embedding of structs instead of interfaces.
Still, it confuses me a lot. Especially the state where the Interface is Nil :(.