Get pointer to struct field (reflection)

[Guillermo Estrada]

Hey Gophers! I'm trying to generate flags based on struct fields via reflection and I reached an issue with Type Assertion and pointers to struct value, I hope someone can point me in the right direction. This is to do some kind of Task manager library to make my life easier writing binaries.

Example: (note this is a short example without Kind() checks and without PkgPath check for only Exported fields)

type Test struct {

  Name   string    `cmd:"Usage message"`

}

// This will be the tested Method for the command line

func (t Test) Hello() {

 fmt.Println("Hello", t.Name)

}

func GenerateFlags(cmd interface{}) error {

  cmdType := reflect.TypeOf(cmd)

    // Validate Kind() is reflection.Struct and proceed

  for i := 0; i < cmdType.NumField(); i++ {

    field := cmdType.Field(i) // Get the StructField

    // Validate field.PkgPath == "" so we know its an Exported field

    cmdValue := reflect.ValueOf(cmd)    // In case we need to use the reflection.Value

    value := cmdValue.FieldByName(field.Name).Interface()   // get the interface to work with

    switch value.(type) {

      //  Switch cases for each of supported flags values

      case string:

       // Below the type assertion as we give the value of the flag is OK, my problem is trying to get a pointer of the valid type via reflection

flag.StringVar(/*HERE IS MY ISSUE*/,field.Name, value.(string) , field.Tag.Get("cmd"))

     }

  }

}

func main() {

    GenerateFlags(Test{"World"}) // Default value for the flag...

    flag.Parse()

    //More code to execute the method given on the Args...

    // something like $program test:hello -name "Peter"

}

I've tried every way I have found in the docs to 1) Get the pointer and 2) Make the Type assertion to *string, apparently I can do either or, but not both. There must be an easy syntax problem I do not know about. 

Hope someone can help.

Kudos!

[Dave Cheney]

Please try

switch v := v.(type) {

case string:

     // v is a string in this scope

default:

    // v is some unknown type

}

http://play.golang.org/p/RT94FkNy2Y

Thanks

Dave

[Guillermo Estrada]

On Tuesday, August 25, 2015 at 12:52:28 AM UTC-5, Dave Cheney wrote:

Please try

switch v := v.(type) {

case string:

     // v is a string in this scope

default:

    // v is some unknown type

}

Thanks Dave, that works! Doing the type assertion at the switch resolves the issue I had, but does the type asserted value after the assigment points to the original struct field? (or is it a copy in case of ints and such) I mean sending the address to the flag function will be able to change its content after flag.Parse(), correct?

value := cmdValue.FieldByName(field.Name).Interface()

switch value := value.(type) {

case string:

flag.StringVar(&value, strings.ToLower(field.Name), value, field.Tag.Get("cmd"))

}

[Carlos Castillo]

No, the value you generated will not be a pointer to the original struct field, and therefore you will not be able to change your original struct. You will need to through reflection get the address of the field (ie: field.Addr()), but to do that, the field, and the struct must be addressable, which requires you pass a pointer to the struct to your reflection function. http://blog.golang.org/laws-of-reflection

http://play.golang.org/p/1iYNkDLTdN

[Guillermo Estrada]

No, the value you generated will not be a pointer to the original struct field, and therefore you will not be able to change your original struct. You will need to through reflection get the address of the field (ie: field.Addr()), but to do that, the field, and the struct must be addressable, which requires you pass a pointer to the struct to your reflection function. http://blog.golang.org/laws-of-reflection

http://play.golang.org/p/1iYNkDLTdN

Gracias Carlos! Passing the struct pointer and then getting the element works as expected in your example, swicthing the type assertion on the pointer is also a nice way to go. 

[Matt Joiner]

I implemented just this for flag parsing here: https://github.com/anacrolix/tagflag.