Access struct property by name

Question

Here is a simple go program that is not working :

package main import "fmt"  type Vertex struct {     X int     Y int }  func main() {     v := Vertex{1, 2}     fmt.Println(getProperty(&v, "X")) }  func getProperty(v *Vertex, property string) (string) {     return v[property] } 

Error:

prog.go:18: invalid operation: v[property] (index of type *Vertex)

What I want is to access the Vertex X property using its name. If I do v.X it works, but v["X"] doesn't.

Can someone tell me how to make this work ?

Answer 1

Most code shouldn't need this sort of dynamic lookup. It's inefficient compared to direct access (the compiler knows the offset of the X field in a Vertex structure, it can compile v.X to a single machine instruction, whereas a dynamic lookup will need some sort of hash table implementation or similar). It's also inhibits static typing: the compiler has no way to check that you're not trying to access unknown fields dynamically, and it can't know what the resulting type should be.

But... the language provides a reflect module for the rare times you need this.

package main  import "fmt" import "reflect"  type Vertex struct {     X int     Y int }  func main() {     v := Vertex{1, 2}     fmt.Println(getField(&v, "X")) }  func getField(v *Vertex, field string) int {     r := reflect.ValueOf(v)     f := reflect.Indirect(r).FieldByName(field)     return int(f.Int()) } 

There's no error checking here, so you'll get a panic if you ask for a field that doesn't exist, or the field isn't of type int. Check the documentation for reflect for more details.