douliao5942
2016-08-18 13:05
浏览 283
已采纳

v.Type()。Elem()和v.Elem()。Type()有什么区别?

In the following code, both options seem to allocate the same resource

func Allocate(v interface{}) error {
    rv := reflect.ValueOf(v)
    if rv.IsNil() {
        return errors.New("Value of v is nil")
    }
    s0 := reflect.New(rv.Type().Elem())
    s1 := reflect.New(rv.Elem().Type())

    return errors.New(fmt.Sprintf("What's the diff? %v %v", s0, s1))
}

If there's no difference in this specific example, an example illustrating the difference will be great. Also what's the preferable option in this specific case when trying to allocate for an interface.

Edit: reflect.DeepEqual(s0, s1) returns false. I think rv.Elem().Type() has a problem dealing with zero values so maybe rv.Type().Elem() is preferred.

Playground

图片转代码服务由CSDN问答提供 功能建议

在以下代码中,这两个选项似乎分配了相同的资源

   func Allocate(v interface {})错误{
 rv:= reflect.ValueOf(v)
,如果rv.IsNil(){
返回error.New(“ v的值为nil”)
  } 
 s0:=反射.New(rv.Type()。Elem())
 s1:=反射.New(rv.Elem()。Type())
 
返回错误。New(fmt。  Sprintf(“差异是什么?%v%v”,s0,s1))
} 
   
 
 

如果在此特定示例中没有区别,则说明 差异会很大。 在尝试分配接口时,在这种特定情况下,还有什么更好的选择。

编辑:reflect.DeepEqual(s0,s1)返回false。 我认为rv.Elem()。Type()在处理零值时遇到问题,因此可能首选rv.Type()。Elem()。

游乐场

  • 点赞
  • 写回答
  • 关注问题
  • 收藏
  • 邀请回答

1条回答 默认 最新

  • douzou7012 2016-08-18 15:33
    已采纳

    There is no difference if v is a non-nil pointer type.

    s := "hello"
    rv := reflect.ValueOf(&s)
    fmt.Println(rv.Type().Elem() == rv.Elem().Type()) // prints "true"
    

    Here are some examples where rv.Type().Elem() and rv.Elem().Type()) are different:

    // nil pointer
    var p *string
    rv := reflect.ValueOf(p)
    fmt.Println(rv.Type().Elem()) // prints "string"
    fmt.Println(rv.Elem().Type()) // panic: call of reflect.Value.Type on zero Value
    
    // interface value
    var i interface{} = "hello"
    rv := reflect.ValueOf(&i).Elem()
    fmt.Println(rv.Type())        // prints "interface {}"
    fmt.Println(rv.Elem().Type()) // prints "string"
    fmt.Println(rv.Type().Elem()) // panic: Elem of invalid type
    

    If rv.Type().Elem() is used in Allocate, then the nil check can be removed and the function will work with nil pointer values.

    The call reflect.DeepEqual(s0, s1) returns false because the ptr fields in the values are different. DeepEqual compares unsafe pointers as simple values, not as pointers. This example might help explain what's going on:

    v := "hello"
    rv := reflect.ValueOf(&v)
    s0 := reflect.New(rv.Type().Elem())
    s1 := reflect.New(rv.Elem().Type())
    s2 := reflect.New(rv.Type().Elem())
    s3 := reflect.New(rv.Elem().Type())
    fmt.Println(reflect.DeepEqual(s0, s1)) // prints "false"
    fmt.Println(reflect.DeepEqual(s0, s2)) // prints "false"
    fmt.Println(reflect.DeepEqual(s1, s3)) // prints "false"
    fmt.Println(reflect.DeepEqual(s2, s3)) // prints "false"
    fmt.Println(reflect.DeepEqual(s0.Interface(), s1.Interface())) // prints "true"
    fmt.Println(reflect.DeepEqual(s0.Interface(), s2.Interface())) // prints "true"
    fmt.Println(reflect.DeepEqual(s1.Interface(), s3.Interface())) // prints "true"
    fmt.Println(reflect.DeepEqual(s2.Interface(), s3.Interface())) // prints "true"
    

    As you can see, the reflect.Value comparisons are all false, even when created using the same sequence of calls.

    点赞 打赏 评论

相关推荐 更多相似问题