How to Safely Convert a Nil Interface to a Pointer in Go?

Nil Interface to Pointer Conversion Error in GoLang

In GoLang, attempting to convert a nil interface to a pointer of a specific type, as illustrated below, triggers an error:

type Nexter interface {
    Next() Nexter
}

type Node struct {
    next Nexter
}

func (n *Node) Next() Nexter {...}

func main() {
    var p Nexter

    var n *Node
    fmt.Println(n == nil) // true
    n = p.(*Node) // error
}

This error occurs because a static-type variable of interface type (e.g., Nexter) can hold values of various dynamic types, including nil. While it's possible to convert a nil value to a pointer using (*Node)(nil), type assertion as used in the example cannot be applied because:

x.(T)

requires x to be non-nil and of type T. In this case, p may contain values of various types or nil.

Instead, explicit checks can be employed:

if p != nil {
    n = p.(*Node) // succeeds if p contains a *Node value
}

Alternatively, the "comma-ok" form provides a safer option:

if n, ok := p.(*Node); ok {
    fmt.Printf("n=%#v\n", n)
}

Using the "comma-ok" form allows handling of both nil and non-nil interface values without triggering a panic.

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