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 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?

Answer 1

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 2

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,
    }
}

Answer 3

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.

Answer 4

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 5

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.

Answer 6

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.)