Q:

Size in bytes of the content of a slice

I am trying to send some data to OpenGl.  Sending an array is easy thanks to Sizeof:
array := [...]Whatever {lots of data}
array_ptr := gl.Pointer(&array[0])
array_size := gl.Sizeiptr(unsafe.Sizeof(array))
gl.BufferData(gl.ARRAY_BUFFER, array_size, array_ptr, gl.STATIC_DRAW)

I would like to use slices instead of arrays because the size of my 3D models is not known at compile time.
How do I retrieve the size, in bytes, of the content of a slice?  I thought of this:
size := uintptr(len(slice)) * unsafe.Sizeof(slice[0])

but it is not very general.  Indeed, I need to know the underlying type of the slice in order for this to work, and this supposes that all the elements of the array have the same size.
I could also loop over the whole slice and add all the sizes of every element.  It's not very fast.
I am ready to always keep the len(s)==cap(s), can that help me?
Edit: implementation of the proposed solution using runtime reflection
package main

import "fmt"
import "reflect"

func ElemSize(container interface{}) uintptr {
    return reflect.TypeOf(container).Elem().Size()
}
func ElemSizeVerbose(container interface{}) uintptr {
    t := reflect.TypeOf(container)
    e := t.Elem()
    s := e.Size()
    fmt.Println(t, e, s)
    return s
}
func main() {
    a := [...]int8{2, 3, 5, 7, 11} // Array
    s := []int64{2, 3, 5, 7, 11} // Slice
    z := []int32{} // Even empty things
    ElemSizeVerbose(a) // [5]int8 int8 1
    ElemSizeVerbose(s) // []int64 int64 8
    ElemSizeVerbose(z) // []int32 int32 4
}

A:

In a slice or array, every element is always the same size. Therefore, your example will work as long as len(s) > 0. In other words, as long as the slice has at least one element in it. Otherwise it will panic.
To avoid the need of having an element in the slice, I recommend using the following:
 uintptr(len(s)) * reflect.TypeOf(s).Elem().Size()

