Optional parameters with defaults in Go struct constructors

Question

I've found myself using the following pattern as a way to get optional parameters with defaults in Go struct constructors:

package main

import (
    "fmt"
)

type Object struct {
    Type int
    Name string
}

func NewObject(obj *Object) *Object {
    if obj == nil {
        obj = &Object{}
    }
    // Type has a default of 1
    if obj.Type == 0 {
        obj.Type = 1
    }
    return obj
}

func main() {
    // create object with Name="foo" and Type=1
    obj1 := NewObject(&Object{Name: "foo"})
    fmt.Println(obj1)

    // create object with Name="" and Type=1
    obj2 := NewObject(nil)
    fmt.Println(obj2)

    // create object with Name="bar" and Type=2
    obj3 := NewObject(&Object{Type: 2, Name: "foo"})
    fmt.Println(obj3)
}

Is there a better way of allowing for optional parameters with defaults?

Answer 1

Dave Cheney offered a nice solution to this where you have functional options to overwrite defaults:

https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis

So your code would become:

package main

import (
    "fmt"
)

type Object struct {
    Type int
    Name string
}

func NewObject(options ...func(*Object)) *Object {
    // Setup object with defaults 
    obj := &Object{Type: 1}
    // Apply options if there are any
    for _, option := range options {
        option(obj)
    }
    return obj
}

func WithName(name string) func(*Object) {
    return func(obj *Object) {
        obj.Name = name
    }
}

func WithType(newType int) func(*Object) {
    return func(obj *Object) {
        obj.Type = newType
    }
}

func main() {
    // create object with Name="foo" and Type=1
    obj1 := NewObject(WithName("foo"))
    fmt.Println(obj1)

    // create object with Name="" and Type=1
    obj2 := NewObject()
    fmt.Println(obj2)

    // create object with Name="bar" and Type=2
    obj3 := NewObject(WithType(2), WithName("foo"))
    fmt.Println(obj3)
}

https://play.golang.org/p/pGi90d1eI52

Answer 2

The approach seems reasonable to me. However, you have a bug. If I explicitly set Type to 0, it will get switched to 1.

My suggested fix: Use a struct literal for the default value: http://play.golang.org/p/KDNUauy6Ie

Or perhaps extract it out: http://play.golang.org/p/QpY2Ymze3b