Golang: Assigning a value to struct member that is a pointer

Question

I'm trying to assign a value to a struct member that is a pointer, but it gives "panic: runtime error: invalid memory address or nil pointer dereference" at runtime...

package main

import (
    "fmt"
    "strconv"
)

// Test
type stctTest struct {
    blTest *bool
}

func main() {

    var strctTest stctTest
    *strctTest.blTest = false

    fmt.Println("Test is " + strconv.FormatBool(*strctTest.blTest))

}

The runtime error seems to come from the assignment of the value with *strctTest.blTest = false , but why? How do I set it to false?

Here is a nice explanation of pointers in Go: dave.cheney.net/2017/04/26/… — fhe
– fhe, Commented May 5, 2017 at 19:01

Zan Lynx · Accepted Answer · 2017-05-06 02:42:04Z

18

Why is it an error? Because a pointer only points. It doesn't create anything to point AT. You need to do that.

How to set it to false? This all depends on WHY you made it a pointer.

Is every copy of this supposed to point to the same bool? Then it should be allocated some space in a creation function.

func NewStruct() *strctTest {
    bl := true
    return &strctTest{
        blTest: &bl,
     }
}

Is the user supposed to point it at a boolean of his own? Then it should be set manually when creating the object.

func main() {
    myBool := false
    stctTest := strctTest{
        blTest: &myBool
    }

    fmt.Println("Test is " + strconv.FormatBool(*strctTest.blTest))

}

edited May 6, 2017 at 2:42

answered May 5, 2017 at 18:40

Zan Lynx

54.7k11 gold badges85 silver badges135 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Bart Silverstrim Over a year ago

My attempt to try the first method gave a compiler error ("invalid pointer type *bool for composite literal") but the second method worked for my purposes. Thank you!

Zan Lynx Over a year ago

@BartSilverstrim Ah well I didn't test it. I guess I should have.

Zan Lynx Over a year ago

@BartSilverstrim I fixed my first example.

Andrei Lupuleasa · Accepted Answer · 2020-03-02 08:10:21Z

4

You could also do something like:

package main

import (
    "fmt"
    "strconv"
)

// Test
type stctTest struct {
    blTest *bool
}

func main() {

    strctTest := stctTest{
        blTest: &[]bool{true}[0],
    }

    fmt.Println("Test is " + strconv.FormatBool(*strctTest.blTest))

}

https://play.golang.org/p/OWSosQhrUql

answered Mar 2, 2020 at 8:10

Andrei Lupuleasa

2,7623 gold badges18 silver badges36 bronze badges

1 Comment

Prashant Over a year ago

this is exactly what I was looking for .. Thanks :)

JT. · Accepted Answer · 2017-05-07 18:05:28Z

1

Another way you can think of it is the zero value of a boolean is false.

This is not as clear but another way to do it.

https://play.golang.org/p/REbnJumcFi

I would recommend a New() func that returns a reference to a initialized struct type.

answered May 7, 2017 at 18:05

JT.

4821 gold badge3 silver badges9 bronze badges

2 Comments

nosequeldeebee Over a year ago

This didn't really answer the OP's question. He asked how to assign a value to a bool, not how to make a new memory allocation. In your example you should add how to assign a value to it. See play.golang.org/p/iqOD5hLcPPp

scosman Over a year ago

If you're using a bool pointer instead of a bool, it's hopefully because you want to support three values true/false/nil, not 2 with default.

Alexander Köb · Accepted Answer · 2022-11-24 16:15:55Z

0

Following up on JTs Answer, I also would recommend using the new function as such:

package main

import (
    "fmt"
    "strconv"
)

// Test
type stctTest struct {
    blTest *bool
}

func main() {

    strctTest := &stctTest{
        blTest: new(bool),
    }

    *strctTest.blTest = true

    fmt.Println("Test is " + strconv.FormatBool(*strctTest.blTest))
}

After initializing the memory with new(), you can directly assign a value to the de-referenced pointer. This way you do not need to use another variable to get the address from.

https://go.dev/play/p/BmekoTalQVh

answered Nov 24, 2022 at 16:15

Alexander Köb

9761 gold badge9 silver badges19 bronze badges

Collectives™ on Stack Overflow

Golang: Assigning a value to struct member that is a pointer

4 Answers 4

3 Comments

1 Comment

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

3 Comments

1 Comment

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related