How to set default values in Go structs

Question

There are multiple answers techniques to the below question    How to set default values to golang structs  How to initialize structs in golang   I have a couple of answers but further discussion is required

User · Accepted Answer

Force a method to get the struct  the constructor way   From this post   A good design is to make your type unexported  but provide an exported constructor function like NewMyType   in which you can properly initialize your struct   type  Also return an interface type and not a concrete type  and the interface should contain everything others want to do with your value  And your concrete type must implement that interface of course   This can be done by simply making the type itself unexported  You can export the function NewSomething and even the fields Text and DefaultText  but just don t export the struct type something   Another way to customize it for you own module is by using a Config struct to set default values  Option 5 in the link   Not a good way though

User · Answer

There is a way of doing this with tags  which allows for multiple defaults      Assume you have the following struct  with 2 default tags default0 and default1     type A struct      I int     default0  3  default1  42      S string  default0  Some String     default1  Some Other String          Now it s possible to Set the defaults    func main      ptr     amp A    Set ptr   default0   fmt Printf  ptr I  d ptr S  s n   ptr I  ptr S     ptr I 3 ptr S Some String     Set ptr   default1   fmt Printf  ptr I  d ptr S  s n   ptr I  ptr S     ptr I 42 ptr S Some Other String        Here s the complete program in a playground    If you re interested in a more complex example  say with slices and maps  then  take a look at creasty defaultse

User · Answer

One problem with option 1 in answer from  Victor Zamanian is that if the type isn t exported then users of your package can t declare it as the type for function parameters etc  One way around this would be to export an interface instead of the struct e g   package candidate    Exporting interface instead of struct type Candidate interface       Struct is not exported type candidate struct       Name string     Votes uint32    Defaults to 0      We are forced to call the constructor to get an instance of candidate func New name string  Candidate       return candidate name  0      enforce the default value here     Which lets us declare function parameter types using the exported Candidate interface  The only disadvantage I can see from this solution is that all our methods need to be declared in the interface definition  but you could argue that that is good practice anyway

User · Answer

type Config struct       AWSRegion                               string  default  us-west-2

User · Answer

From https   golang org doc effective go html composite literals   Sometimes the zero value isn t good enough and an initializing constructor is necessary  as in this example derived from package os        func NewFile fd int  name string   File         if fd  lt  0           return nil               f    new File        f fd   fd       f name   name       f dirinfo   nil       f nepipe   0       return f

User · Answer

One possible idea is to write separate constructor function    Something is the structure we work with type Something struct        Text string       DefaultText string        NewSomething create new instance of Something func NewSomething text string  Something      something    Something      something Text   text    something DefaultText    default text     return something

[go] How to set default values in Go structs

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