[struct] How to check for an empty struct?

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?

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")
  } 
}

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

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)
}

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().

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 := &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...