On Sun, Oct 24, 2021 at 6:54 PM jlfo...@berkeley.edu <jlforr...@berkeley.edu>
wrote:
> I'm now aware that that I could use
>
> len(([unsafe.Sizeof(T{})][0]byte{}))
>
> But this seems overly complicated. Plus, I don't see why
> this is unsafe. Please advise.
>
Sizeof is unsafe because it returns the number of bytes of the underlying
object, including the overhead such as padding or headers. For slices, it
returns the size of the header, not the size of the underlying array.
Sizeof also means you are probably bypassing the Go type system.
>
> Jon
>
>
> On Sunday, October 24, 2021 at 5:14:44 PM UTC-7 kortschak wrote:
>
>> You can use len to determine the sizeof a struct.
>>
>> https://play.golang.org/p/f0x8p_04lP1 ;)
>>
>>
>> On Sun, 2021-10-24 at 16:44 -0700, jlfo...@berkeley.edu wrote:
>> > Thanks for the replies.
>> >
>> > I had been trying to translate the trick from C into Go where you can
>> > find how many structures are in an initialized array of structures
>> > by dividing the size of the array by the size of one structure. As
>> > I've learned, not only isn't this possible, because you can't get
>> > the size of one structure, but also because it isn't necessary.
>> > Instead, using a slice of structures rather than an array, I can just
>> > do "len(structslice)".
>> > That gives me what I need.
>> >
>> > But, I still have some questions about the responses. First, I think
>> > the expected value of len(struct) should be its size, in bytes,
>> > like with a string. Are there any examples of problems this would
>> > cause? I don't understand why this has to be
>> > unsafe. (It could even be done at compile time). I also don't
>> > understand the comment about recursive calls. Since it's possible
>> > to assign one structure to another I would think that the structure
>> > length is known. Since I'm new to Go I'm probably
>> > not aware of the finer points that would make this not so.
>> >
>> > Cordially,
>> > Jon Forrest
>> >
>> > On Sunday, October 24, 2021 at 11:29:58 AM UTC-7
>> > filipdimi...@gmail.com wrote:
>> > > len() works on indexed types - arrays, slices, maps, strings. It
>> > > also works on buffered channels but we can consider that a
>> > > sequence. I don't consider a struct a sequence. It's non-obvious
>> > > what the behaviour would be. Both of these sound reasonable: len()
>> > > should be the memory size of the struct like sizeof in C/C++ *or*
>> > > it should be the sum of the lengths of its (sequence) members.
>> > >
>> > > The first case is way too low-level for Go, because it belongs to
>> > > an unsafe operation so that's an easy no, ... and it already exists
>> > > as unsafe.Sizeof().
>> > >
>> > > The second case (len being the sum of its members' lengths) would
>> > > require recursive calls, and almost surely infinite cycles as we
>> > > eventually get to pointers.
>> > >
>> > > On Sunday, October 24, 2021 at 8:11:37 PM UTC+2
>> > > jlfo...@berkeley.edu wrote:
>> > > > I noticed that the len() function doesn't take a struct as an
>> > > > argument (see below).
>> > > > This is a big surprise. Can someone shed some light on why this
>> > > > restriction exists?
>> > > >
>> > > > Cordially,
>> > > > Jon Forrest
>> > > >
>> > > > ----------
>> > > > package main
>> > > >
>> > > > import "fmt"
>> > > >
>> > > > var s struct {
>> > > > i1 int
>> > > > i2 int
>> > > > }
>> > > >
>> > > > func main() {
>> > > > fmt.Printf("len(s) = %d\n", len(s)) // -- invalid argument
>> > > > s (type struct { i1 int; i2 int }) for len
>> > > > }
>>
>>
>> --
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