why does golang nil pointer dereference happen in submethods-CodePudding

I have this code

package main

import "fmt"

type Foo struct {
    Bar string
}

func (f *Foo) level4() {
    fmt.Printf("Bar = %s\n", f.Bar)
}

func (f *Foo) level3() {
    f.level4() // panics here, 2 levels down
}

func (f *Foo) level2() {
    f.level3()
}

func (f *Foo) level1() {
    f.level2()
}

type FooWrapper struct {
    foo *Foo
}

func main() {
    w := FooWrapper{}
    w.foo.level1() // expected it to panic here, since foo is nil
}

As expected, running this gives

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x47f454]

However, I expected the nil pointer dereference to happen at w.foo.level1(), since foo is nil. Instead, it calls levels 1, 2 and 3 and panics there. Why is this the case?

playground link

CodePudding user response：

Why is this the case?

Because w.foo.level1() is valid statement and similarly f.level2(), f.level3(), f.level4() are also a valid statement.

try this

func (f *Foo) level1() {
    println("Hello Go")
    f.level2()
}

it will print Hello Go and call f.level2() and also see last call level 4

func (f *Foo) level4() {
    println("Hello Go 4")
    fmt.Printf("Bar = %s\n", f.Bar) 
}

it will print Hello Go 4 but panic on next line it is showing the trace or you can say origin of error

CodePudding user response：

You did not initialize the FooWrapper in your main function correctly. Your foo field with type *Foo is nil inside w variable type of FooWrapper because you have not assigned a value for that and the default value for a pointer type variable is nil in Go

Simple correct initialization as follows

    w := FooWrapper{
        foo: &Foo{
            Bar: "bar",
        },
    }

output :

Bar = bar

run in playground