CODEWITHSUNDEEP
go
Ado Ren
Ado Ren

Reputation: 4394

How to check if generic type is nil

I have a generic function. I want to check if parameter provided is nil (and prevent it).

What is the best way to do it?

Here is a sample code:

func main() {
    Hello(1)
}

func Hello[T any](id T) {
    if reflect.ValueOf(id).IsNil() {
        panic("id cannot be nil")
    }
    fmt.Printf("Hello %v\n", id)
}

That code panics.

Upvotes: 5

Views: 5236

Answers (3)

Gabe M
Gabe M

Reputation: 2741

Adapted from anon's answer — checking for nils

func isNil[T any](t T) bool {
v := reflect.ValueOf(t)
kind := v.Kind()
// Must be one of these types to be nillable
return (kind == reflect.Ptr ||
    kind == reflect.Interface ||
    kind == reflect.Slice ||
    kind == reflect.Map ||
    kind == reflect.Chan ||
    kind == reflect.Func) &&
    v.IsNil()
}

Upvotes: 1

Sean F
Sean F

Reputation: 4605

For pointers this works:

func isNil[T comparable](arg T) bool {
    var t T
    return arg == t
}

https://go.dev/play/p/G9K3i28qGFX

Upvotes: 0

anon
anon

Reputation: 21

The only types that can be nil are chan, func, interface, map, pointer, and slice. Check for one of those types before calling IsNil

func Hello[T any](id T) {
    if v := reflect.ValueOf(id); (v.Kind() == reflect.Ptr ||
        v.Kind() == reflect.Interface ||
        v.Kind() == reflect.Slice ||
        v.Kind() == reflect.Map ||
        v.Kind() == reflect.Chan ||
        v.Kind() == reflect.Func) && v.IsNil() {
        panic("id cannot be nil")
    }
    fmt.Printf("Hello %v\n", id)
}

Upvotes: 2

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