Thread overview
bitmanip : read does not accept my array slice
Dec 26
Dennis
Dec 27
Dennis
December 26
I was trying to translate this kind of C code to D:

void calc(unsigned char *buf) {
  (...)
  res = read_u32_be(&buf[i]);
}

So I tried this:

import std.bitmanip : read, Endian;
void calc(ubyte[] buf) {
  (...)
  res = read!(uint, Endian.bigEndian)(buf[i..$]);
}

But then I get this error:
template std.bitmanip.read cannot deduce function from argument types !(uint, cast(Endian)0)(ubyte[])

The weird thing is that the following does compile, despite having the same types:

ubyte[] tmp = buf[i..$];
res = read!(uint, Endian.bigEndian)(tmp);

Why does this happen?


December 26
On Tuesday, 26 December 2017 at 21:45:29 UTC, Dennis wrote:
> I was trying to translate this kind of C code to D:
>
> void calc(unsigned char *buf) {
>   (...)
>   res = read_u32_be(&buf[i]);
> }
>
> So I tried this:
>
> import std.bitmanip : read, Endian;
> void calc(ubyte[] buf) {
>   (...)
>   res = read!(uint, Endian.bigEndian)(buf[i..$]);
> }
>
> But then I get this error:
> template std.bitmanip.read cannot deduce function from argument types !(uint, cast(Endian)0)(ubyte[])
>
> The weird thing is that the following does compile, despite having the same types:
>
> ubyte[] tmp = buf[i..$];
> res = read!(uint, Endian.bigEndian)(tmp);
>
> Why does this happen?

read takes a ref argument so it can change the value of it (like it tries to assign buf[i..$] = something then) which doesn't work for slices via ref arguments.

Instead what you would want to use is peek:

void calc(ubyte[] buf) {
  size_t i = 0;
  (...)
  res = buf.peek!uint(&i);
}

BigEndian is default btw, you don't need to specify that but you can if you want.

What peek does now is first dereferencing your pointer there to get the current value of i to look where to read and then increment the value by T.sizeof (uint.sizeof here, which is 4). With this you can read multiple successing values without doing anything with i. You can also pass a normal int value instead of a pointer which will just peek without advancing the value
December 27
Ah, so it's about lvalues an rvalues, not the type of the range. Makes sense now.

On Tuesday, 26 December 2017 at 22:33:54 UTC, WebFreak001 wrote:
> BigEndian is default btw, you don't need to specify that but you can if you want.

Dealing with Endianness bugs has been such a pain, I like to be explicit about it ;)

On Tuesday, 26 December 2017 at 22:33:54 UTC, WebFreak001 wrote:
> What peek does now is first dereferencing your pointer there to get the current value of i to look where to read and then increment the value by T.sizeof (uint.sizeof here, which is 4). With this you can read multiple successing values without doing anything with i. You can also pass a normal int value instead of a pointer which will just peek without advancing the value

That function is more applicable here, thanks!