Using reflect, how do you set the value of a struct field?
Solution 1
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.
Solution 2
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
cc young
semi-retired, living in Sihanoukville, Cambodia open for: Postgres consulting IT training in Cambodia
Updated on May 01, 2021Comments
-
cc young about 3 years
having a rough time working with struct fields using
reflect
package. in particular, have not figured out how to set the field value.type t struct { fi int; fs string } var r t = t{ 123, "jblow" } var i64 int64 = 456
getting Name of field i - this seems to work
var field = reflect.TypeOf(r).Field(i).Name
getting value of field i as a) interface{}, b) int - this seems to work
var iface interface{} = reflect.ValueOf(r).Field(i).Interface()
var i int = int(reflect.ValueOf(r).Field(i).Int())
setting value of field i - try one - panic
reflect.ValueOf(r).Field(i).SetInt( i64 )
panic: reflect.Value·SetInt using value obtained using unexported field
assuming it did not like field names "id" and "name", so renamed to "Id" and "Name"
a) is this assumption correct?
b) if correct, thought not necessary since in same file / package
setting value of field i - try two (with field names capitalized ) - panic
reflect.ValueOf(r).Field(i).SetInt( 465 )
reflect.ValueOf(r).Field(i).SetInt( i64 )
panic: reflect.Value·SetInt using unaddressable value
Instructions below by @peterSO are thorough and high quality
Four. this works:
reflect.ValueOf(&r).Elem().Field(i).SetInt( i64 )
he documents as well that the field names must be exportable (begin with capital letter)
-
cc young almost 13 yearsGive Up! somewhere in there is an answer, but four hours of work on the json pkg have not yielded it to me. re the reflect pkg, pulling info is pretty straightforward, but setting data requires some of black magic for which I would love to see a simple example somewhere!
-
cc young almost 13 yearsthanks! now that I've read peterSO's notes, this makes perfect sense. I was using foo, not &foo, so could not be changed, and was unsure what Elem() was about.
-
cc young almost 13 yearsOutstanding! if you're ever in Thailand please let me treat you to a beer or two or three! thank you very much
-
danmux about 11 yearsGreat practical example, this article completely demystified it for me golang.org/doc/articles/laws_of_reflection.html
-
Sarath Sadasivan Pillai almost 9 yearsAwesome example.Here is playground sample of the same code play.golang.org/p/RK8jR_9rPh
-
wvxvw about 7 yearsThis doesn't set a struct field. This sets a field in a pointer to struct. Obviously, this is not answering the question OP asked.
-
U Avalos about 6 yearsomg, why is go so painful to use. Grumble, grumble grumble. Repeat the above for all the different data types and you see what i mean
-
g10guang over 5 yearsReflect is a terrible thing. Programmer can visit the unexport field just as fmt.Printf can show us the unexport field. Thanks golang team, they won't allow programmers to change the unexport field. Or else something ridiculous will come.
-
Carson about 3 yearsSupplement: The necessary condition for CanSet to return true is that the attribute of the struct must have an uppercase prefix.