How to convert/assign a value with type *string to type *wrapperspb.StringValue when the *string value is nil?

Issue

I am trying to assign a value with type *string to a variable with type *wrapperspb.StringValue. However, when the *string is nil, it triggers an error (please see the comments in the snipped code to see what kind of error).

Here is a simplified version of my code:

// You can edit this code!
// Click here and start typing.
package main

import (
    "fmt"

    "google.golang.org/protobuf/types/known/wrapperspb"
)

func main() {
    var var1 *wrapperspb.StringValue
    var1 = &wrapperspb.StringValue{Value: "test1"}
    fmt.Println("var1:")
    fmt.Println(var1)

    var var2 *string
    fmt.Println("var2:")
    fmt.Println(var2)

    //var1 = var2 // it says "Cannot use 'var2' (type *string) as the type *wrapperspb.StringValue"
    //var1 = wrapperspb.String(*var2) // it says panic: runtime error: invalid memory address or nil pointer dereference
    //fmt.Println("var1 with var2 value:")
    //fmt.Println(var1)
}

Does anyone know how to properly handle the conversion/assignment?

Here is a golang playground: https://go.dev/play/p/5JBfU0oEIC-

Solution

If your var2 string pointer is nil, you should also leave the var1 *wrapperspb.StringValue pointer nil as well. Methods of wrapperspb.StringValue handles if itself is the nil pointer. So "convert" it like this:

if var2 == nil {
    var1 = nil
} else {
    var1 = wrapperspb.String(*var2)
}

Testing it:

for i := 0; i < 2; i++ {
    var var1 *wrapperspb.StringValue

    var var2 *string
    if i == 0 {
        s := "test"
        var2 = &s
    }

    if var2 == nil {
        var1 = nil
    } else {
        var1 = wrapperspb.String(*var2)
    }
    fmt.Printf("var2: %v\n", var2)
    fmt.Printf("var1: %v\n", var1)
    fmt.Printf("%q\n", var1.GetValue())
}

This will output (try it on the Go Playground):

var2: 0xc00009e470
var1: value:"test"
"test"
var2: <nil>
var1: <nil>
""

Answered By – icza

Answer Checked By – Dawn Plyler (GoLangFix Volunteer)

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