How can i get a struct size?

[18126...@163.com]

 my code like below, use binary.Size to get my struct size ,but it returns -1, not my required size. is there anybody help me ?

type Req struct{
       header Head
       body Body
}
type Head struct{
       length uint32
       sequence uint32
}
type Body struct{
       entitySize uint32
       entity []Entity
}
type Entity struct{
       id uint32
       status uint16
}

func main() {
       e1 := Entity{}
       e2 := Entity{}
       v := Req{}
       v.body.entity = append(v.body.entity, e1)
       v.body.entity = append(v.body.entity, e2)
       fmt.Println(binary.Size(v))
       // this output is -1.
}

[Dave Cheney]

encoding/binary.Size can only report the encoded size under very strict circumstances. You probably want something like unsafe.Sizeof

https://golang.org/pkg/unsafe/#Sizeof

[andrey mirtchovski]

as per the documentation, binary.Size works only on things which are
"a fixed-size value or a slice of fixed-size values, or a pointer to
such data". you have a struct there, called "Body", which is not fixed
size as it contains a slice. you wouldn't have a problem calling
binary.Size on an []Entity, but not on a Body:

https://play.golang.org/p/k37YLbnRje

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[Roberto Zanotto]

reflect.TypeOf(Req{}).Size()

https://golang.org/pkg/reflect/#Type

[18126...@163.com]

I try "unsafe.Sizeof(Req{})" and "reflect.TypeOf(Req{}).Size" to calculate the 'Req' struct size by byte , they all return 24, if i append the 3rd Entity,  it's still 24.  

It's not the result i want 30, i think it should be increase 6 bytes with every increased Entity. 

I 'am a newer in go program , i expect to get a most convenient way to calculate the Req struct size by byte, can anybody give me some ideas , many thanks.

在 2016年5月13日星期五 UTC+8下午12:11:38，18126...@163.com写道：

[Michael Jones]

The structure member entity (“entity []Entity”) is a slice, that is, it is a handle describing the location and size of a array of Entities. The size of the handle is not increased as the array size increases.

The kind of size you are thinking of is “how many bytes in the Body structure and how many active bytes in the particular backing array referenced by the entity member/” This is a fine question to ask, but it is not the answer to the question “how many bytes in the Body structure?”

You seem to want:  SIZE(Body) + len(someParticularBody.entity)*SIZE(Entity)

— 

Michael Jones, CEO  •  mic...@wearality.com  •  +1 650 656-6989 

Wearality Corporation  •  289 S. San Antonio Road  •  Los Altos, CA 94022

[18126...@163.com]

The following code is working, but it seems not very flexiable, it must know the slice position in whole body, then call len(silce) and sizeof(Entiy)

// this function is from modified binary size() function

func sizeof(t reflect.Type) int {
       switch t.Kind() {
       case reflect.Array:
              fmt.Println("reflect.Array")
              if s := sizeof(t.Elem()); s >= 0 {
                     return s * t.Len()
              }

       case reflect.Struct:
              fmt.Println("reflect.Struct")
              sum := 0
              for i, n := 0, t.NumField(); i < n; i++ {
                     s := sizeof(t.Field(i).Type)
                     if s < 0 {
                            return -1
                     }
                     sum += s
              }
              return sum

       case reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64,
              reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
              reflect.Float32, reflect.Float64, reflect.Complex64, reflect.Complex128:
              fmt.Println("reflect.int")
              return int(t.Size())
       case reflect.Slice:

                     //modified place
              fmt.Println("reflect.Slice:", sizeof(t.Elem()))
              return 0
       }

       return -1

}

type Req struct{
       header Head
       body Body
}
type Head struct{
       length uint32
       sequence uint32
}
type Body struct{
       entitySize uint32
       entity []Entity
}
type Entity struct{
       id uint32
       status uint16
}

func main() {
       e1 := Entity{}
       e2 := Entity{}

       e3 := Entity{}

       v := Req{}
       v.body.entity = append(v.body.entity, e1)
       v.body.entity = append(v.body.entity, e2)

       v.body.entity = append(v.body.entity, e3)

       base := sizeof(reflect.TypeOf(v))
       variable := len(v.body.entity ) * sizeof(reflect.TypeOf(e3))
       fmt.Println(base + variable)

}

在 2016年5月13日星期五 UTC+8下午2:52:29，Michael Jones写道：

[Dave Cheney]

Can I ask a different question. If you could compute the size of Req, what would you do with that information?

[刘翰林]

I get the length of the body ,translate it to bytes, then use socket to send bytes . Do that,I don't need to count when I translate uint32 to bytes .

发自我的 iPhone

> 在 2016年5月13日，16:29，Dave Cheney <da...@cheney.net> 写道：
>
> Can I ask a different question. If you could compute the size of Req, what would you do with that information?
>

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[刘翰林]

The reflect.Type.Size() can't calculate slice struct.

发自我的 iPhone

在 2016年5月13日，15:43，Roberto Zanotto <roby...@gmail.com> 写道：

Why don't you just use reflect.Type.Size()? Do you need to chase the pointer in the slice headers recursively?

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[Ian Lance Taylor]

On Fri, May 13, 2016 at 3:32 AM, 刘翰林 <18126...@163.com> wrote:
> I get the length of the body ,translate it to bytes, then use socket to send bytes . Do that,I don't need to count when I translate uint32 to bytes .

Assuming you want to translate to bytes using encoding/binary, decode
the value into bytes, then ask for the length of what you got.

Is there a reason you want to get the length first?

Ian

>> 在 2016年5月13日，16:29，Dave Cheney <da...@cheney.net> 写道：
>>
>> Can I ask a different question. If you could compute the size of Req, what would you do with that information?
>>
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>
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[Konstantin Khomoutov]

On Fri, 13 May 2016 06:32:34 -0700
Ian Lance Taylor <ia...@golang.org> wrote:

> On Fri, May 13, 2016 at 3:32 AM, 刘翰林 <18126...@163.com> wrote:
> > I get the length of the body ,translate it to bytes, then use
> > socket to send bytes . Do that,I don't need to count when I
> > translate uint32 to bytes .
>
> Assuming you want to translate to bytes using encoding/binary, decode
> the value into bytes, then ask for the length of what you got.
>
> Is there a reason you want to get the length first?

Supposedly to not buffer the serialized data.

[刘翰林]

When I get the head,the total bytes length can be used for body bytes read on the peer socket. Or is there another way?

发自我的 iPhone

[Roberto Zanotto]

If you can, use something like gob

https://golang.org/pkg/encoding/gob/#example__basic

It's a binary encoding and it takes care of all those things for you.

[刘翰林]

Thanks for your advice,I will try it:)