• Dennis (19/19) Dec 26 2017 I was trying to translate this kind of C code to D:
• WebFreak001 (18/37) Dec 26 2017 read takes a ref argument so it can change the value of it (like
• Dennis (7/15) Dec 26 2017 Ah, so it's about lvalues an rvalues, not the type of the range.

Dennis <dkorpel gmail.com> writes:

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?

WebFreak001 <d.forum webfreak.org> writes:

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

Dennis <dkorpel gmail.com> writes:

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!