void[] vs ubyte[] - differences?

Jul 16

H. S. Teoh

Jul 17

Nick Treleaven

Jul 17

Nick Treleaven

Jul 17

Dukc

Jul 18

Nick Treleaven

import std; void main() { void[] a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 0x12345678]; void[] b = cast(ubyte[])[0, 1, 2, 3, 4, 5, 6, 7, 8]; ubyte[] c = cast(ubyte[])(cast(void[])[0, 1, 2, 3, 4, 5, 6, 7, 8,0x12345678]); ubyte[] d = [0, 1, 2, 3, 4, 5, 6, 7, 8]; void[] e = cast(ubyte[])a;// same result as a void[] f = cast(void[]) a;// same result as a writeln(a); writeln(b); writeln(c); writeln(d); writeln(e); writeln(f); writeln(void.sizeof); }

[0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 5, 0, 0, 0, 6, 0, 0, 0, 7, 0, 0, 0, 8, 0, 0, 0, 120, 86, 52, 18] [0, 1, 2, 3, 4, 5, 6, 7, 8] [0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 5, 0, 0, 0, 6, 0, 0, 0, 7, 0, 0, 0, 8, 0, 0, 0, 120, 86, 52, 18] [0, 1, 2, 3, 4, 5, 6, 7, 8] [0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 5, 0, 0, 0, 6, 0, 0, 0, 7, 0, 0, 0, 8, 0, 0, 0, 120, 86, 52, 18] [0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 5, 0, 0, 0, 6, 0, 0, 0, 7, 0, 0, 0, 8, 0, 0, 0, 120, 86, 52, 18] 1 //it also appears to expose the platform's endianness

On Wed, Jul 16, 2025 at 01:29:01PM +0000, z via Digitalmars-d-learn wrote: [...] > I also see this in the language documentation : > ``` > A void array cannot be indexed. > ``` > But i can slice it just fine in DMD 2.111... Probably an oversight. > Is this by design or is there a hole in the language specification? By extension what the differences between `void`, `ubyte`, `void[]`, `ubyte[]` really? The language documentation doesn't appear to be explaining everything. [...] One important difference between void[] and ubyte[] is that void[] may contain GC-scannable pointers, whereas ubyte[] is assumed to be pure data. This may be important if you're storing live pointers in a buffer; if the buffer was allocated as a ubyte[], the GC might not scan it for pointers and may incorrectly collect live objects. (Though I'm not 100% sure the current GC implementation actually does this.) Also, void means "no type", which is completely different from "ubyte", which means an unsigned 8-bit integer. Some introspection operations like is(...) expressions return void when given an invalid expression, whereas returning ubyte means the expression evaluates to an 8-bit unsigned byte type. T -- What's a "hot crossed bun"? An angry rabbit.

On Wednesday, 16 July 2025 at 13:29:01 UTC, z wrote:

I also see this in the language documentation :

A void array cannot be indexed.

But i can slice it just fine in DMD 2.111...

Is this by design or is there a hole in the language specification?

The sentence before says:

Array indices in slicing operations are interpreted as byte indices

And the example shows slicing:

    void[] arr = data1;            // OK, int[] implicit converts to void[].
...
    arr[0..4] = [5];               // Assign first 4 bytes to 1 int element

Slicing a void array is useful as it can refer to multiple bytes. Indexing a void array is not really useful because if the array is only holding bytes, then why not use byte[] instead? If it's not a byte, then indexing it is probably wrong anyway (either by design, or because dealing with a single byte at a time is inefficient).

By extension what the differences between void, ubyte, void[], ubyte[] really? The language documentation doesn't appear to be explaining everything.
Thanks in advance

The docs explain the difference between the two array types. Perhaps there should be an entry for void in the types page. A void value cannot be accessed. void.sizeof is 1 so that a void array can have length equivalent to the number of bytes in the array.