Constructors in Go

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 really OOP in the traditional sense these aren't true objects and it has no constructors.

I have noticed the init method but that is at the package level. Is there something else similar that can be used at the struct level?

If not what is the accepted best practice for this type of thing in Go?

There are some equivalents of constructors for when the zero values can't make sensible default values or for when some parameter is necessary for the struct initialization.

Supposing you have a struct like this :

type Thing struct {
	Name  string
	Num   int
}

then, if the zero values aren't fitting, you would typically construct an instance with a NewThing function returning a pointer :

func NewThing(someParameter string) *Thing {
	p := new(Thing)
	p.Name = someParameter
	p.Num = 33 // <- a very sensible default value
	return p
}

When your struct is simple enough, you can use this condensed construct :

func NewThing(someParameter string) *Thing {
	return &Thing{someParameter, 33}
}

If you don't want to return a pointer, then a practice is to call the function makeThing instead of NewThing :

func makeThing(name string) Thing {
	return Thing{name, 33}
}

Reference : Allocation with new in Effective Go.

There are actually two accepted best practices:

1. Make the zero value of your struct a sensible default. (While this looks strange to most people coming from "traditional" oop it often works and is really convenient).
2. Provide a function func New() YourTyp or if you have several such types in your package functions func NewYourType1() YourType1 and so on.

Document if a zero value of your type is usable or not (in which case it has to be set up by one of the New... functions. (For the "traditionalist" oops: Someone who does not read the documentation won't be able to use your types properly, even if he cannot create objects in undefined states.)

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 I finished 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.

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.

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

another way is;

package person

type Person struct {
    Name string
    Old  int
}

func New(name string, old int) *Person {
    // set only specific field value with field key
    return &Person{
        Name: name,
    }
}

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.

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
}

Golang is not OOP language in its official documents. All fields of Golang struct has a determined value(not like c/c++), so constructor function is not so necessary as cpp. If you need assign some fields some special values, use factory functions. Golang's community suggest New.. pattern names.

I am new to go. I have a pattern taken from other languages, that have constructors. And will work in go.

1. Create an init method.
2. Make the init method an (object) once routine. It only runs the first time it is called (per object).

func (d *my_struct) Init (){
    //once
    if !d.is_inited {
        d.is_inited = true
        d.value1 = 7
        d.value2 = 6
    }
}

3. Call init at the top of every method of this class.

This pattern is also useful, when you need late initialisation (constructor is too early).

Advantages: it hides all the complexity in the class, clients don't need to do anything.

Disadvantages: you must remember to call Init at the top of every method of the class.