How can I use reflect.Value.Call() with a nil input?

Question

I want to invoke a function in Go, using reflect.Value.Call on a method value, and pass nil as a parameter. See the code below for an illustration.

I have tried using reflect.ValueOf(nil) and reflect.Value{} in the input array, but the first panics because nil has no value; the second panics when I pass it to Call, because it is a Zero reflect.Value.

Note that, as the code illustrates, it is certainly possible to pass nil to a function without reflection, including when that argument is the receiver. The question is: Is it possible to invoke a func using reflect.Value.Call, passing one of those parameters as nil?

You can build and run the code below at: http://play.golang.org/p/x9NXMDHWdM

package main

import "reflect"

type Thing struct{}

var (
    thingPointer = &Thing{}
    typ          = reflect.TypeOf(thingPointer)
)

func (t *Thing) DoSomething() {
    if t == nil {
        println("t was nil")
    } else {
        println("t was not nil")
    }
}

func normalInvokation() {
    thingPointer.DoSomething()
    // prints "t was not nil"
    t := thingPointer
    t = nil
    t.DoSomething()
    // prints "t was nil"
}

func reflectCallNonNil() {
    m, _ := typ.MethodByName("DoSomething")
    f := m.Func
    f.Call([]reflect.Value{reflect.ValueOf(&Thing{})})
    // prints "t was not nil"
}

func reflectCallNil() {
    // m, _ := typ.MethodByName("DoSomething")
    // f := m.Func
    // f.Call(???)
    // how can I use f.Call to print "t was nil" ?
}

func main() {
    normalInvokation()
    reflectCallNonNil()
    reflectCallNil()
}

Answer 1

One option is to simply tell reflect that you mean a nil pointer to Thing :

func reflectCallNonNil() {
    m, _ := typ.MethodByName("DoSomething")
    f := m.Func
    f.Call([]reflect.Value{reflect.ValueOf((*Thing)(nil))})

}

Also, you can use reflect.Zero which gives you a "zero value" for a type. So nil in this context is the zero value of pointer to Thing .

Thus, doing this will work:

func reflectCallNil() {
    m, _ := typ.MethodByName("DoSomething")
    f := m.Func
    f.Call([]reflect.Value{reflect.Zero(reflect.TypeOf(&Thing{}))})
}