Nil interfaces vs nil concrete values

Context

An interface value is a pair (type, value). It is nil only if both type and value are nil. If you assign a nil concrete pointer to an interface, the interface itself is not nil because it holds a type.

Example

Demonstrate the difference.

Code example

package main

import "fmt"

type MyError struct{}

func (e *MyError) Error() string {
    return "my error"
}

func returnsError() error {
    var e *MyError = nil
    return e // returns a non-nil interface (type is *MyError, value is nil)
}

func main() {
    var i interface{} = nil
    fmt.Println("i is nil:", i == nil) // true

    err := returnsError()
    fmt.Println("err is nil:", err == nil) // false
    fmt.Printf("err type: %T, value: %v\n", err, err)

    // To check if the concrete value is nil:
    if err == nil {
        fmt.Println("This won't print")
    } else if err != nil {
        fmt.Println("err is not nil, but the concrete value may be nil")
    }
}

Output

i is nil: true
err is nil: false
err type: *main.MyError, value: <nil>
err is not nil, but the concrete value may be nil

Nil interfaces vs nil concrete values

Context

Example

Code example

Output

Useful links