你可以使用 reflect.Value.Call() 方法来调用一个 nil 输入。

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英文:

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

问题

我想在Go中调用一个函数,使用reflect.Value.Call在方法值上,并将nil作为参数传递。请参考下面的代码示例。

我尝试在输入数组中使用reflect.ValueOf(nil)reflect.Value{},但第一个会引发panic,因为nil没有值;第二个在我将其传递给Call时引发panic,因为它是一个Zero reflect.Value。

请注意,正如代码所示,可以在不使用反射的情况下将nil传递给函数,包括当该参数是接收器时。问题是:是否可以使用reflect.Value.Call调用一个函数,将其中一个参数设置为nil?

你可以在以下网址构建和运行下面的代码: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()
}
英文:

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()
}

答案1

得分: 6

一种选择是直接告诉reflect你指的是一个指向Thing的空指针:

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

另外,你可以使用reflect.Zero,它会给你一个类型的"零值"。所以在这个上下文中,nil就是指向Thing的指针的零值。

因此,这样做也是可以的:

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

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{}))})
}

huangapple
  • 本文由 发表于 2014年9月6日 21:11:02
  • 转载请务必保留本文链接:https://go.coder-hub.com/25700630.html
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