Go – How to loop through the fields in a Golang struct to get and set values in an extensible way

goreflection

I have a struct Person.

type Person struct {
    Firstname string       
    Lastname  string       
    Years     uint8       
}

Then I have two instances of this struct, PersonA and PersonB.

PersonA := {"", "Obama", 6}
PersonB := {"President", "Carter", 8}

I want to write a function that copies the values from PersonA to PersonB given some condition for each field (i.e. non-empty). I know how to do this by hard-coding the field names, but I want a function that works even if I change the Person struct.

I know Go reflections is helpful, but the issue is getting and setting the values requires knowing the types, if you want to use something like SetInt. But is there is a "simple" way to do this?

** Javascript analogy **
In Javascript, you could just do a (for property in someObject) to loop through.

(for propt in personA) {
  if personA[propt] != "" {
    // do something
    personB[propt] = personA[propt]
  }
}

Options I've ruled out:

Keeping track of the fields in each struct in a map, then using a combination of FieldByName and the collection of Set* functions in the reflect pkg.
Creating a loop through the fields of Person manually (below). Because I want to do this type of "update" for many other structs (School, Animals, etc…)
```
if PersonA.Firstname != "" {
  PersonB.Firstname = PersonA.Firstname 
}
```
…
```
if PersonA.Years != "" {
  PersonB.Years = PersonA.Years 
}
```

The question below gets me half-way there, but still isn't extensible to all structs for which I want to utilize this "update" function.

in golang, using reflect, how do you set the value of a struct field?

** Other Helpful Links **
GoLang: Access struct property by name

Best Answer

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")
    }
}

Related Solutions

Go – Using reflect, how do you set the value of a struct field

The Go json package marshals and unmarshals JSON from and to Go structures.

Here's a step-by-step example which sets the value of a struct field while carefully avoiding errors.

The Go reflect package has a CanAddr function.

func (v Value) CanAddr() bool

CanAddr returns true if the value's address can be obtained with Addr. Such values are called addressable. A value is addressable if it is an element of a slice, an element of an addressable array, a field of an addressable struct, or the result of dereferencing a pointer. If CanAddr returns false, calling Addr will panic.

The Go reflect package has a CanSet function, which, if true, implies that CanAddr is also true.

func (v Value) CanSet() bool

CanSet returns true if the value of v can be changed. A Value can be changed only if it is addressable and was not obtained by the use of unexported struct fields. If CanSet returns false, calling Set or any type-specific setter (e.g., SetBool, SetInt64) will panic.

We need to make sure we can Set the struct field. For example,

package main

import (
    "fmt"
    "reflect"
)

func main() {
    type t struct {
        N int
    }
    var n = t{42}
    // N at start
    fmt.Println(n.N)
    // pointer to struct - addressable
    ps := reflect.ValueOf(&n)
    // struct
    s := ps.Elem()
    if s.Kind() == reflect.Struct {
        // exported field
        f := s.FieldByName("N")
        if f.IsValid() {
            // A Value can be changed only if it is 
            // addressable and was not obtained by 
            // the use of unexported struct fields.
            if f.CanSet() {
                // change value of N
                if f.Kind() == reflect.Int {
                    x := int64(7)
                    if !f.OverflowInt(x) {
                        f.SetInt(x)
                    }
                }
            }
        }
    }
    // N at end
    fmt.Println(n.N)
}

Output:
42
7

If we can be certain that all the error checks are unnecessary, the example simplifies to,

package main

import (
    "fmt"
    "reflect"
)

func main() {
    type t struct {
        N int
    }
    var n = t{42}
    fmt.Println(n.N)
    reflect.ValueOf(&n).Elem().FieldByName("N").SetInt(7)
    fmt.Println(n.N)
}

BTW, Go is available as open source code. A good way to learn about reflection is to see how the core Go developers use it. For example, the Go fmt and json packages. The package documentation has links to the source code files under the heading Package files.

Go – in golang, general function to load http form data into a struct

You need to make switches for the general case, and load the different field types accordingly. This is basic part.

It gets harder when you have slices in the struct (then you have to load them up to the number of elements in the form field), or you have nested structs.

I have written a package that does this. Please see:

http://www.gorillatoolkit.org/pkg/schema

Best Answer

Related Solutions

Go – Using reflect, how do you set the value of a struct field

Go – in golang, general function to load http form data into a struct

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