JuniorTheoryCommon

What does the program print?

package main

import "fmt"

type Rect struct{ W, H float64 }

func main() {
    var r *Rect
    var s any = r
    fmt.Println(r == nil, s == nil, s.(*Rect) == nil)
}

true false true

Expecting true true true: an interface value is nil only when both its type and its value are empty, and s already holds the type *Rect.
Expecting a panic at s.(*Rect): the assertion succeeds because the dynamic type is *Rect, and it returns the nil pointer, so == nil is true.

Why can a function that returns error hand back a non-nil error that holds a nil pointer?
What would s == nil print right after var s any, before the assignment?

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JuniorTheoryCommon

What does the program print?

package main

import "fmt"

type Rect struct{ W, H float64 }

func main() {
    var r *Rect
    var s any = r
    fmt.Println(r == nil, s == nil, s.(*Rect) == nil)
}

true false true

Expecting true true true: an interface value is nil only when both its type and its value are empty, and s already holds the type *Rect.
Expecting a panic at s.(*Rect): the assertion succeeds because the dynamic type is *Rect, and it returns the nil pointer, so == nil is true.

Why can a function that returns error hand back a non-nil error that holds a nil pointer?
What would s == nil print right after var s any, before the assignment?

Back to questions Open topic in roadmap →Answer it yourself — Quiz