How to check for an empty struct?

struct go
Michael picture Michael · Feb 11, 2015 · Viewed 139.6k times · Source

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?

Answer

Cerise Limón picture Cerise Limón · Feb 11, 2015

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")
}

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