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

Question

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.

This seems to work:

package main

import (
    "fmt"
    "reflect"
)

type Foo struct {
    Number int
    Text string
}

func main() {
    foo := Foo{123, "Hello"}

    fmt.Println(int(reflect.ValueOf(foo).Field(0).Int()))

    reflect.ValueOf(&foo).Elem().Field(0).SetInt(321)

    fmt.Println(int(reflect.ValueOf(foo).Field(0).Int()))
}

Prints:

123
321