Call a struct field with a string in golang

Question

I want to call a field of my structure with a string, so I checked out and I saw that I need to use the reflection package. Here's my code:

package main

import (
    "fmt"
    "reflect"
)


type cmd struct{
    echo func() (string,error)
}



func main() {
    cmd:=cmd{
        echo : func () (string,error){
            return "test", nil
        },
    }
    fmt.Println(reflect.ValueOf(&cmd).MethodByName("echo").Call([]reflect.Value{}))

}

And when I want to build it, it panics like :

panic: reflect: call of reflect.Value.Call on zero Value

goroutine 1 [running]:
reflect.flag.mustBe(...)
        c:/go/src/reflect/value.go:208
reflect.Value.Call(0x0, 0x0, 0x0, 0xc0000c7f50, 0x0, 0x0, 0x0, 0x0, 0xc000086058)
        c:/go/src/reflect/value.go:319 +0x174

I do not understand the []reflect.Value{}, why do I need to use this on the line :

reflect.ValueOf(&cmd).MethodByName("echo").Call([]reflect.Value{})

Hope someone can help ! :D

Chris

Eklavya · Accepted Answer · 2020-04-06 10:27:50Z

3

cmd.echo is not a method. It's a field of function type. Access it using Value.FieldByName().

And only exported fields can be accessed, rename it to Echo.

And you have to pass a non-pointer cmd, or if you pass &cmd, you have to call Value.Elem().

Working example:

type cmd struct {
    Echo func() (string, error)
}

cmd := cmd{
    Echo: func() (string, error) {
        return "test", nil
    },
}
fmt.Println(reflect.ValueOf(cmd).FieldByName("Echo").Call([]reflect.Value{}))

This outputs (try it on the Go Playground):

[test <error Value>]

Also note that if the function does not have parameters, you pass nil to Value.Call():

reflect.ValueOf(cmd).FieldByName("Echo").Call(nil)

edited Apr 6, 2020 at 10:27

Eklavya

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answered Apr 6, 2020 at 9:58

icza

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2 Comments

Zouglou Over a year ago

Thanks man ! And if I want to have a parameter in my function, do I pass this parameter in the {} of the reflect.Value or in the array of the Call ? Cuz I try to pass "test" for example and in the reflect.Value it says : cannot use "test" (type string) as type reflect.Value in slice literal. When I pass it in the array of Call, it says : invalid array bound "test". I don't understand how to pass a parameter like "test" for my function...

icza Over a year ago

@Zouglou If the function has a string parameter, this is how you can call that: reflect.ValueOf(cmd).FieldByName("Echo").Call([]reflect.Value{reflect.ValueOf("someparam")})

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