Constructors in Go

Question

I have a struct and I would like it to be initialised with some sensible default values.

Typically, the thing to do here is to use a constructor but since go isn\'t

Answer 1

There are no default constructors in Go, but you can declare methods for any type. You could make it a habit to declare a method called "Init". Not sure if how this relates to best practices, but it helps keep names short without loosing clarity.

package main

import "fmt"

type Thing struct {
    Name string
    Num int
}

func (t *Thing) Init(name string, num int) {
    t.Name = name
    t.Num = num
}

func main() {
    t := new(Thing)
    t.Init("Hello", 5)
    fmt.Printf("%s: %d\n", t.Name, t.Num)
}

The result is:

Hello: 5

Answer 2

If you want to emulate ___.new() syntax you can do something along the lines of:

type Thing struct {
    Name string
    Num int
}
type Constructor_Thing struct {}
func (c CThing) new(<<CONSTRUCTOR ARGS>>) Thing {
  var thing Thing
  //initiate thing from constructor args
  return thing
}
var cThing CThing

func main(){
  var myThing Thing
  myThing = cThing.new(<<CONSTRUCTOR ARGS>>)
  //...
}

Granted, it is a shame that Thing.new() cannot be implemented without CThing.new() also being implemented (iirc) which is a bit of a shame...

Answer 3

Go has objects. Objects can have constructors (although not automatic constructors). And finally, Go is an OOP language (data types have methods attached, but admittedly there are endless definitions of what OOP is.)

Nevertheless, the accepted best practice is to write zero or more constructors for your types.

As @dystroy posted his answer before me finishing this answer, let me just add an alternative version of his example constructor, which I would probably write instead as:

func NewThing(someParameter string) *Thing {
    return &Thing{someParameter, 33} // <- 33: a very sensible default value
}

The reason I want to show you this version is that pretty often "inline" literals can be used instead of a "constructor" call.

a := NewThing("foo")
b := &Thing{"foo", 33}

Now *a == *b.

Answer 4

I like the explanation from this blog post:

The function New is a Go convention for packages that create a core type or different types for use by the application developer. Look at how New is defined and implemented in log.go, bufio.go and cypto.go:

log.go

// New creates a new Logger. The out variable sets the
// destination to which log data will be written.
// The prefix appears at the beginning of each generated log line.
// The flag argument defines the logging properties.
func New(out io.Writer, prefix string, flag int) * Logger {
    return &Logger{out: out, prefix: prefix, flag: flag}
}

bufio.go

// NewReader returns a new Reader whose buffer has the default size.
func NewReader(rd io.Reader) * Reader {
    return NewReaderSize(rd, defaultBufSize)
}

crypto.go

// New returns a new hash.Hash calculating the given hash function. New panics
// if the hash function is not linked into the binary.
func (h Hash) New() hash.Hash {
    if h > 0 && h < maxHash {
        f := hashes[h]
        if f != nil {
            return f()
        }
    }
    panic("crypto: requested hash function is unavailable")
}

Since each package acts as a namespace, every package can have their own version of New. In bufio.go multiple types can be created, so there is no standalone New function. Here you will find functions like NewReader and NewWriter.

Answer 5

In Go, a constructor can be implemented using a function that returns a pointer to a modified structure.

type Colors struct {
    R   byte
    G   byte
    B   byte
}

// Constructor
func NewColors (r, g, b byte) *Colors {
    return &Color{R:r, G:g, B:b}
}

For weak dependencies and better abstraction, the constructor does not return a pointer to a structure, but an interface that this structure implements.

type Painter interface {
    paintMethod1() byte
    paintMethod2(byte) byte
}

type Colors struct {
    R byte
    G byte
    B byte
}

// Constructor return intreface
func NewColors(r, g, b byte) Painter {
    return &Color{R: r, G: g, B: b}
}

func (c *Colors) paintMethod1() byte {
    return c.R
}

func (c *Colors) paintMethod2(b byte) byte {
    return c.B = b
}

Answer 6

If you want to force the factory function usage, name your struct (your class) with the first character in lowercase. Then, it won't be possible to instantiate directly the struct, the factory method will be required.

This visibility based on first character lower/upper case work also for struct field and for the function/method. If you don't want to allow external access, use lower case.