CODEWITHSUNDEEP
goreflection
Jonathan Hall
Jonathan Hall

Reputation: 79594

How to initialize struct pointer via reflection

My goal is to take a nil pointer to a struct (but could be any type), passed as an interface{}, and initialize a struct in its place.

My test code (playground link) is:

package main

import (
    "fmt"
    "reflect"
)

type Foo struct {
    Foo string
}

func main() {
    var x *Foo
    var y interface{} = x
    fmt.Printf("Before: %#v\n", y)
    fmt.Printf("Goal: %#v\n", interface{}(&Foo{}))

    rv := reflect.ValueOf(y)
    rv.Set(reflect.New(rv.Type().Elem()))

    fmt.Printf("After: %#v\n", y)
}

I hope the code is self-documenting. But the goal is essentially to convert y which begins as an uninitialized pointer to Foo, ((*main.Foo)(nil)) into a pointer to an initialized (zero-value) instance ofFoo: &main.Foo{Foo:""}. But I'm getting reflect.Value.Set using unaddressable value. I don't understand why the value I'm attempting to set is unaddressable. I've spent the day reading through the source code to the standard library JSON unmarshaler, and other SO posts, but am still clearly overlooking something.

If I peel away the outer interface{}:

rv := reflect.ValueOf(y).Elem() // Remove the outer interface{}
rv.Set(reflect.New(rv.Type().Elem()))

the error becomes reflect: call of reflect.Value.Type on zero Value.

Upvotes: 0

Views: 2158

Answers (3)

Thundercat
Thundercat

Reputation: 120951

Try this:

var x *Foo
var y interface{} = x
fmt.Printf("Before: %#v\n", y)
fmt.Printf("Goal: %#v\n", interface{}(&Foo{}))

// Must take address of y to set it. Dereference with Elem() to get value for y
rv := reflect.ValueOf(&y).Elem()

// Interface element type is *main.Foo, dereference with Elem() to get main.Foo
t := rv.Elem().Type().Elem()
rv.Set(reflect.New(t))
fmt.Printf("After: %#v\n", y)

playground example

You can also assign y instead of setting it via reflection:

var x *Foo
var y interface{} = x
fmt.Printf("Before: %#v\n", y)
fmt.Printf("Goal: %#v\n", interface{}(&Foo{}))
rv := reflect.ValueOf(y)
t := rv.Type().Elem()
y = reflect.New(t).Interface()
fmt.Printf("After: %#v\n", y)

playground example

Upvotes: 4

TehSphinX
TehSphinX

Reputation: 7430

In your example rv.CanAddr() is false and consequently you cannot set it (rv.CanSet() is false).

Problem: if you have a pointer to nil it is not addressable. A pointer to a pointer to nil (or a pointer to an interface) however is addressable.

You can get your example to work in different ways:

  • initialize x to be non-nil (probably not the point here)

  • set y to a new Value with reflect.New(..)

    func main() {
    var x *Foo
    var y interface{} = x
    fmt.Printf("Before: %#v\n", y)
    fmt.Printf("Goal: %#v\n", interface{}(&Foo{}))

    rv := reflect.ValueOf(y)
    y = reflect.New(rv.Type().Elem()).Interface()

    fmt.Printf("After: %#v\n", y)
}

Upvotes: 1

Adrian
Adrian

Reputation: 46442

Without knowing your actual use case, it's hard to say what solution you require, however this works for your example:

var x *Foo
var y interface{} = x
rv := reflect.ValueOf(y)
y = reflect.New(rv.Type().Elem()).Interface()

Upvotes: 1

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