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reflect.Value.MethodByName is not returning zero value when not found #39307

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kstenerud opened this issue May 28, 2020 · 3 comments
Closed

reflect.Value.MethodByName is not returning zero value when not found #39307

kstenerud opened this issue May 28, 2020 · 3 comments

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@kstenerud
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@kstenerud kstenerud commented May 28, 2020

What version of Go are you using (go version)?

play.golang.org

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

play.golang.org

What did you do?

https://play.golang.org/p/Rnjm3-DT5Il

package main

import (
	"fmt"
	"reflect"
)

func main() {
	v := reflect.ValueOf(1)
	m := v.MethodByName("String")
	if !m.IsValid() {
		fmt.Println("Invalid")
	}
	if m.IsZero() {
		fmt.Println("Zero")
	}
}

What did you expect to see?

Zero

What did you see instead?

Invalid
panic: reflect: call of reflect.Value.IsZero on zero Value

goroutine 1 [running]:
reflect.Value.IsZero(0x0, 0x0, 0x0, 0x1)
	/usr/local/go-faketime/src/reflect/value.go:1111 +0x4b6
main.main()
	/tmp/sandbox851150891/prog.go:14 +0xf1

According to https://golang.org/pkg/reflect/#Value.MethodByName "It returns the zero Value if no method was found."

@ianlancetaylor
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@ianlancetaylor ianlancetaylor commented May 28, 2020

The zero Value is not the value for which the IsZero method returns true; it is the Value for which the IsValid method returns false. See the docs for IsValid, which say "It returns false if v is the zero Value." See also the docs for Value, which say "The zero Value represents no value. Its IsValid method returns false, its Kind method returns Invalid, its String method returns "", and all other methods panic. "

@kstenerud
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@kstenerud kstenerud commented May 29, 2020

That's incredibly confusing, having a "zero" value that panics on IsZero(). Surely there's a more elegant way to structure the API?

@ianlancetaylor
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@ianlancetaylor ianlancetaylor commented May 29, 2020

Well, perhaps there is. Too late now, though, per https://golang.org/doc/go1compat.

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