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

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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

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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  amp Person          Name  name

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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

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I am new to go  I have another pattern taken from other languages  that have constructors  And will work in go    Create an init method  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            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

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There are actually two accepted best practices    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   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

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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  amp 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  amp 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

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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

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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  amp Thing someParameter  33      lt - 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     amp Thing  foo   33    Now  a     b

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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     lt - 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  amp 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

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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  lt  lt CONSTRUCTOR ARGS gt  gt   Thing     var thing Thing     initiate thing from constructor args   return thing   var cThing CThing  func main      var myThing Thing   myThing   cThing new  lt  lt CONSTRUCTOR ARGS gt  gt               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

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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  amp 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  gt  0  amp  amp  h  lt  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

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