How to check for an empty struct

Question

I define a struct      type Session struct       playerId string     beehive string     timestamp time Time     Sometimes I assign an empty session to it  because nil is not possible   session   Session      Then I want to check  if it is empty   if session    Session             do stuff        Obviously this is not working  How do I write it

User · Accepted Answer

You can use    to compare with a zero value composite literal because all fields are comparable   if  Session       session        fmt Println  is zero value       playground example  Because of a parsing ambiguity  parentheses are required around the composite literal in the if condition   The use of    above applies to structs where all fields are comparable  If the struct contains a non-comparable field  slice  map or function   then the fields must be compared one by one to their zero values   An alternative to comparing the entire value is to compare a field that must be set to a non-zero value in a valid session  For example  if the player id must be       in a valid session  use   if session playerId             fmt Println  is zero value

User · Answer

As an alternative to the other answers  it s possible to do this with a syntax similar to the way you originally intended if you do it via a case statement rather than an if   session    Session   switch   case Session      session      fmt Println  zero   default      fmt Println  not zero       playground example

User · Answer

Just a quick addition  because I tackled the same issue today  With Go 1 13 it is possible to use the new isZero   method  if reflect ValueOf session  IsZero             do stuff       I didn t test this regarding performance  but I guess that this should be faster  than comparing via reflect DeepEqual

User · Answer

Here are 3 more suggestions or techniques   With an Additional Field  You can add an additional field to tell if the struct has been populated or it is empty  I intentionally named it ready and not empty because the zero value of a bool is false  so if you create a new struct like Session   its ready field will be automatically false and it will tell you the truth  that the struct is not-yet ready  it s empty    type Session struct       ready bool      playerId string     beehive string     timestamp time Time     When you initialize the struct  you have to set ready to true  Your isEmpty   method isn t needed anymore  although you can create one if you want to  because you can just test the ready field itself   var s Session  if  s ready          do stuff  populate s      Significance of this one additional bool field increases as the struct grows bigger or if it contains fields which are not comparable  e g  slice  map and function values    Using the Zero Value of an Existing Field  This is similar to the previous suggestion  but it uses the zero value of an existing field which is considered invalid when the struct is not empty  Usability of this is implementation dependant   For example if in your example your playerId cannot be the empty string     you can use it to test if your struct is empty like this   var s Session  if s playerId                do stuff  populate s  give proper value to playerId      In this case it s worth incorporating this check into an isEmpty   method because this check is implementation dependant   func  s Session  isEmpty   bool       return s playerId           And using it   if s isEmpty            do stuff  populate s  give proper value to playerId      Use Pointer to your struct  The second suggestion is to use a Pointer to your struct   Session  Pointers can have nil values  so you can test for it   var s  Session  if s    nil       s   new Session         do stuff  populate s

User · Answer

Using reflect deepEqual also works  especially when you have map inside the struct  package main  import  fmt  import  time  import  reflect   type Session struct       playerId string     beehive string     timestamp time Time    func  s Session  IsEmpty   bool     return reflect DeepEqual s Session       func main       x    Session     if x IsEmpty         fmt Print  is empty

User · Answer

Keep in mind that with pointers to struct you d have to dereference the variable and not compare it with a pointer to empty struct   session     amp Session   if  Session        session       fmt Println  session is empty       Check this playground   Also here you can see that a struct holding a property which is a slice of pointers cannot be compared the same way

[struct] How to check for an empty struct?

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