How do you loop through the fields in a Golang struct to get and set values in an extensible way?

Question

I have a struct Person.

type Person struct {
    Firstname string       
    Lastname  string       
    Years     uint8       
}

Then I have t

Answer 1

Here is the solution f2.Set(reflect.Value(f)) is the key here

package main

   import (
    "fmt"
    "reflect"
   )

   func main() {
    type T struct {
        A int
        B string
    }
    t := T{23, "skidoo"}
    t2:= T{}
    s := reflect.ValueOf(&t).Elem()
    s2 := reflect.ValueOf(&t2).Elem()
    typeOfT := s.Type()
    fmt.Println("t=",t)
    fmt.Println("t2=",t2)

    for i := 0; i < s.NumField(); i++ {
        f := s.Field(i)
        f2:= s2.Field(i)
        fmt.Printf("%d: %s %s = %v\n", i,
            typeOfT.Field(i).Name, f.Type(), f.Interface())
        fmt.Printf("%d: %s %s = %v\n", i,
            typeOfT.Field(i).Name, f2.Type(), f2.Interface())
        f2.Set(reflect.Value(f))
        fmt.Printf("%d: %s %s = %v\n", i,
            typeOfT.Field(i).Name, f2.Type(), f2.Interface())

    }
    fmt.Println("t=",t)
    fmt.Println("t2=",t2)
}

Output:

t= {23 skidoo}
t2= {0 }
0: A int = 23
0: A int = 0
0: A int = 23
1: B string = skidoo
1: B string = 
1: B string = skidoo
t= {23 skidoo}
t2= {23 skidoo}

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

Answer 2

I don't even know how many ways this can go wrong...

package main

import (
    "fmt"
    "encoding/json"
)

type Serializable interface {
    fromMap(map[string]interface{}) error
    toMap() (map[string]interface{}, error)
}

type Person struct {
    Firstname string       
    Lastname  string       
    Years     uint8       
}

func (p *Person) fromMap(m map[string]interface{}) error {
    b, err := json.Marshal(m)
    if err != nil {
        return err
    }

    if err := json.Unmarshal(b, p); err != nil {
        return err
    }
    return nil
}

func (p Person) toMap() (map[string]interface{}, error) {
    b, err := json.Marshal(p)
    if err != nil {
        return nil, err
    }
    m := map[string]interface{}{}
    if err := json.Unmarshal(b, &m); err != nil {
        return nil, err
    }
    return m, nil
}

func copy(p1 Serializable, p2 Serializable) error {

    m1, err := p1.toMap()
    if err != nil {
        return err
    }

    m2, err := p2.toMap()
    if err != nil {
        return err
    }

    for k := range m1 {
        m2[k] = m1[k]
    }

    if err := p2.fromMap(m2); err != nil {
        return err
    }
    return nil
}

func main() {
    p1 := Person{
        "Mary",
        "Jane",
        26,
    }

    p2 := Person {
        "Random",
        "Lady",
        26,
    }

    if err := copy(&p1, &p2); err != nil {
        fmt.Printf("ERR: %s\n", err.Error())
        return
    }

    fmt.Printf("%v\n", p2)

}

Answer 3

You should use a map[string]interface{} instead, gonna be much faster (although still not as fast as you used the proper logic with actual structs).

package main

import "fmt"

type Object map[string]interface{}

var m = Object{
    "Firstname": "name",
    "Lastname":  "",
    "years":     uint8(10),
}

func main() {
    var cp = Object{}
    for k, v := range m {
        if s, ok := v.(string); ok && s != "" {
            cp[k] = s
        } else if ui, ok := v.(uint8); ok {
            cp[k] = ui
        }
    }
    fmt.Printf("%#v\n", cp)
}

Answer 4

Use reflect.ValueOf() to convert to concrete type. After that you could use reflect.Value.SetString to set the value you want.

structValue := FooBar{Foo: "foo", Bar: 10}
fields := reflect.TypeOf(structValue)
values := reflect.ValueOf(structValue)

num := fields.NumField()

for i := 0; i < num; i++ {
    field := fields.Field(i)
    value := values.Field(i)
    fmt.Print("Type:", field.Type, ",", field.Name, "=", value, "\n")

    switch value.Kind() {
    case reflect.String:
        v := value.String()
        fmt.Print(v, "\n")
    case reflect.Int:
        v := strconv.FormatInt(value.Int(), 10)
        fmt.Print(v, "\n")
    case reflect.Int32:
        v := strconv.FormatInt(value.Int(), 10)
        fmt.Print(v, "\n")
    case reflect.Int64:
        v := strconv.FormatInt(value.Int(), 10)
        fmt.Print(v, "\n")
    default:
        assert.Fail(t, "Not support type of struct")
    }
}

Answer 5

Reflection should be all you need. This seems similar (though not identical) to "deep copy" semantics, which has been implemented at https://godoc.org/github.com/getlantern/deepcopy

You should be able to adapt that to your needs, or at least take some ideas from it.