How can I cast []byte to [8]uint8

Issue

I need to populate a struct that has a member of type [8]uint8. This needs be populated with a byte array of type []byte initialized to length 8. The simplistic approach does not work:

Data:   [8]uint8(RequestFrame(0x180, r)),

gives

cannot convert .. (type []byte) to type [8]uint8

Since both arrays are structurally identical it would be nice if this could be done with casting/assignment rather than copying?

Solution

Background

The problem with your "simplistic approach" is that a slice
(of any type) is a struct-typed value consisting of a pointer
and two integers; the pointer contains the address of the
underlying (backing) data array, and the integers contain
what len() and cap() builtins return for that slice.

In other words, a slice is sort of a view into an array.

Then, in Go, there is no concept of a type cast; there are only
type conversions, and these conversions may only happen between
types with the same underlying representation¹.

Since a slice and an array may not have the same underlying
representation (array is literally a contiguous block of memory
of the size just enough to contain all the array’s elements),
your alleged type conversion may not be legal.

Possible solutions

There are two possible solutions.

The simplest is to just copy the data from the slice’s
backing array into a newly-allocated array:

var (
    src = []byte{1, 2, 3, 4, 5, 6, 7, 8}
    dst [8]uint8
)
copy(dst[:], src[:8])

Note that there exists an inherent disparity between slice an
array types: an array type encodes both the type of its elements
and its length (that is, the length is a part of the type),
while a slice type only encodes the type of its elements
(and may be of any length at runtime).

This means that you might need to have a check before such
copying that makes sure the source slice has exactly 8
elements, that is, len(src) == len(dst).

This invariant may be enforced by some other code, but I think
I’d warn you up front about this: if src has less than 8
elements, the src[:8] expression will panic at runtime,
and if it contains more, then there’s the question of whether
copying just the first 8 of them is exactly what’s needed.

The second approach (admittedly messier) is to just re-use
the underlying array of the slice:

import "unsafe"

var (
    src    = []byte{1, 2, 3, 4, 5, 6, 7, 8}
    dstPtr *[8]uint8
)

if len(src) != len(*dstPtr) {
    panic("boom")
}
dstPtr = (*[8]uint8)(unsafe.Pointer(&src[0]))

Here, we’ve just taken the address of the first element
contained in the slice’s underlying array and peformed
a "dirty" two-phase type-conversion to make the obtained
pointer to be of type *[8]uint8—that is, "an address of
an array of 8 uint8s".

Note two caveats:

  • The resulting pointer now points to
    the same memory block the original slice does.
    It means it’s now possible to mutate that memory both through the
    slice and the pointer we obtained.

  • As soon as you’ll decide to assign the array’s data
    to a variable of type [8]uint8 (and passing it as an argument
    to a function’s parameter of that type), you will dereference
    that pointer (like with *dstPtr), and at that moment
    the array’s data will be copied.

    I’m specifically mentioning this as often people resort
    to hacks like this one to pull the backing array out of
    a slice precisely in an attempt to not copy the memory.

TL;DR

Copy the data (after supposedly verifying the
len(src) == len(dst) invariant holds).

Copying 8 bytes is fast (on a typical 64-bit CPU this will be
a single MOV instruction, or two at most), and the code will
be straightforward.

Only resort to hacks from the second solution when you really
need to optimize on some critical hot path.
In that case, comment the solution extensively and watch for
not accidentally dereferencing your pointer.


¹ There are notable exceptions to this rule:

  • A []byte is type-convertible to string, and vice-versa.
  • A string is type-convertible to []rune, and vice-versa.
  • An int is type-convertible to string (but since Go 1.15 go vet gives a warning about it, and this feature may probably be prohibited in the future).

Answered By – kostix

Answer Checked By – Cary Denson (GoLangFix Admin)

Leave a Reply

Your email address will not be published.