Why Does a Nil Interface to Pointer Conversion Fail in Go?

Nil Interface to Pointer Conversion in Go

In Go, an interface type can hold values of different concrete types that implement it. However, attempting to convert a nil interface to a pointer of a specific type can result in the error "interface conversion: interface is nil, not *". This error occurs because type assertion, which is used to convert an interface to a specific type, requires a non-nil interface value.

To understand this behavior, consider the code snippet from the question:

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: interface conversion: interface is nil, not *main.Node
}

The problem arises when attempting to assign the value of p to n using the type assertion p.(*Node). This assertion fails because the p variable is nil, indicating that it does not hold any concrete value.

Alternatively, the following expression:

n = (*Node)(nil)

succeeds because it explicitly sets the n variable to a nil pointer of type *Node. This is possible because an interface value consists of a pair of (value, dynamic type), and setting an interface value to nil will result in a pair of (nil, nil).

To handle nil interface values, you can check it explicitly:

if p != nil {
    n = p.(*Node)  // Only succeeds if `p` holds a value of type `*Node`
}

Alternatively, you can use the "comma-ok" form of type assertion:

if n, ok := p.(*Node); ok {
    fmt.Printf("n=%#v\n", n)  // Never fails, assigns zero value to `n` if `p` is nil
}

In conclusion, attempting to convert a nil interface to a pointer of a specific type using type assertion will fail, but explicitly setting it to nil will succeed. To handle nil interface values, explicit checks or the "comma-ok" form of type assertion can be used.

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