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.
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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