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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