reflect.ValueOf: unexpected behavior

[LayTsyn]

Go version

go1.23.4 windows/amd64

Output of go env in your module/workspace:

set GODEBUG=
set GOTELEMETRY=local
set GOTELEMETRYDIR=C:\Users\LayTsyn\AppData\Roaming\go\telemetry
set GCCGO=gccgo
set GOAMD64=v1
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=0
set GOMOD=NUL
set GOWORK=
set CGO_CFLAGS=-O2 -g
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-O2 -g
set CGO_FFLAGS=-O2 -g
set CGO_LDFLAGS=-O2 -g
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -fno-caret-diagnostics -Qunused-arguments -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=C:\Users\LayTsyn\AppData\Local\Temp\go-build3606146757=/tmp/go-build -gno-record-gcc-switches

What did you do?

Program1:

package main

import (
	"fmt"
)

type EmptyCustomErr struct {
}

func (ce EmptyCustomErr) Error() string {
	return "EmptyCustomErr"
}

func ReturnMePointerEmpty() error {
	return &EmptyCustomErr{}
}

func main() {
	interfaceWithEmptyPointer := ReturnMePointerEmpty()
	pointerErrEmpty := &EmptyCustomErr{}
	fmt.Println("pointerErrEmpty == interfaceWithPointer: ", pointerErrEmpty == interfaceWithEmptyPointer)

	// fmt.Println(reflect.ValueOf(pointerErrEmpty))
}

Program2:

package main

import (
	"fmt"
	"reflect"
)

type EmptyCustomErr struct {
}

func (ce EmptyCustomErr) Error() string {
	return "EmptyCustomErr"
}

func ReturnMePointerEmpty() error {
	return &EmptyCustomErr{}
}

func main() {
	interfaceWithEmptyPointer := ReturnMePointerEmpty()
	pointerErrEmpty := &EmptyCustomErr{}
	fmt.Println("pointerErrEmpty == interfaceWithPointer: ", pointerErrEmpty == interfaceWithEmptyPointer)

	fmt.Println(reflect.ValueOf(pointerErrEmpty))
}

What did you see happen?

Program1

Output: "pointerErrEmpty == interfaceWithPointer: false"

Program2

Output: "pointerErrEmpty == interfaceWithPointer: true"

What did you expect to see?

I expected to see that regardless of calling the value acquisition method (reflect.ValueOf), the result of the comparison would be the same in the two types of program.

The Go documentation (https://pkg.go.dev/reflect#ValueOf) says:

func ValueOf ¶
func ValueOf(i any) Value
ValueOf returns a new Value initialized to the concrete value stored in the interface i. ValueOf(nil) returns the zero Value.

There is no word here that the transmitted value is being changed. But that seems to be the case. Otherwise, why does calling the information Getter (ValueOf) change the result of the program execution?

[seankhliao]

https://go.dev/doc/faq#zero_size_types

[gabyhelp]

Related Issues

• reflect: MakeFunc functions cannot use the nil error return idiom #6871 (closed)
• reflect: reflect.Value.IsZero results false positive #34108 (closed)
• reflect:documentation for IsValid and IsZero is confusing #34152 (closed)
• reflect: Value.Interface() should return a copy of data for settable values #3134 (closed)
• reflect: Value.Method does not do the same nil checks as the language #32021
• Nil-ness of a pointer is lost when converted to an interface #42306 (closed)
• reflect.Value.Elem is unaddressable for interfaces #17117 (closed)
• reflect.Value.MethodByName is not returning zero value when not found #39307 (closed)

Related Discussions

• [reflect] the type of the return value from reflect.Value.Call() is different
• problem in the reflect package

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