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digitalmars.D - strange bug with unions?

reply "seen" <sycam.inc gmail.com> writes:
I don't know why but when using a template union with a static 
ubyte array i get bizarre results!

module main;

import std.stdio;
private union ByteConverter(T)
{
	ubyte[T.sizeof] bytes;
	T value;
}

ubyte[] toBytes(T)(T from) if (!is(T == const))
{
	ByteConverter!T converter;
	converter.value = from;
	return converter.bytes;
}

T fromBytes(T)(in ubyte[] bytes)
{
	ByteConverter!T converter;
	converter.bytes = bytes;
	return converter.value;
}

void main(string[] args)
{
	writeln(fromBytes!uint(toBytes(34u)));// should print 32 but 
prints 1244564
}

why dose it print 1244564 the array is static size so why is it 
acting like bytes is a pointer not a static array?
May 29 2015
next sibling parent reply "deadalnix" <deadalnix gmail.com> writes:
On Friday, 29 May 2015 at 07:55:55 UTC, seen wrote:

 I don't know why but when using a template union with a static 
 ubyte array i get bizarre results!

 module main;

 import std.stdio;
 private union ByteConverter(T)
 {
 	ubyte[T.sizeof] bytes;
 	T value;
 }

 ubyte[] toBytes(T)(T from) if (!is(T == const))
 {
 	ByteConverter!T converter;
 	converter.value = from;
 	return converter.bytes;
 }

 T fromBytes(T)(in ubyte[] bytes)
 {
 	ByteConverter!T converter;
 	converter.bytes = bytes;
 	return converter.value;
 }

 void main(string[] args)
 {
 	writeln(fromBytes!uint(toBytes(34u)));// should print 32 but 
 prints 1244564
 }

 why dose it print 1244564 the array is static size so why is it 
 acting like bytes is a pointer not a static array?


http://fr.wikipedia.org/wiki/Endianness#Little_endian
May 29 2015
parent "seen" <sycam.inc gmail.com> writes:
On Friday, 29 May 2015 at 08:00:54 UTC, deadalnix wrote:

 On Friday, 29 May 2015 at 07:55:55 UTC, seen wrote:

 I don't know why but when using a template union with a static 
 ubyte array i get bizarre results!

 module main;

 import std.stdio;
 private union ByteConverter(T)
 {
 	ubyte[T.sizeof] bytes;
 	T value;
 }

 ubyte[] toBytes(T)(T from) if (!is(T == const))
 {
 	ByteConverter!T converter;
 	converter.value = from;
 	return converter.bytes;
 }

 T fromBytes(T)(in ubyte[] bytes)
 {
 	ByteConverter!T converter;
 	converter.bytes = bytes;
 	return converter.value;
 }

 void main(string[] args)
 {
 	writeln(fromBytes!uint(toBytes(34u)));// should print 32 but 
 prints 1244564
 }

 why dose it print 1244564 the array is static size so why is 
 it acting like bytes is a pointer not a static array?


 http://fr.wikipedia.org/wiki/Endianness#Little_endian


endianness should not be an issue since I'm not changing machines
and using static array unions is a common way to convert to bytes 
in D.
May 29 2015
prev sibling next sibling parent =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 05/29/2015 12:55 AM, seen wrote:

 I don't know why but when using a template union with a static ubyte
 array i get bizarre results!

 module main;

 import std.stdio;
 private union ByteConverter(T)
 {
      ubyte[T.sizeof] bytes;


That's a fixed-length array (aka static array), which has value semantics.


      T value;
 }

 ubyte[] toBytes(T)(T from) if (!is(T == const))


That function returns a slice to a...


 {
      ByteConverter!T converter;
      converter.value = from;
      return converter.bytes;


... local static array. :(

Replace the return type with

a) auto

b) ubyte[T.sizeof]


 }

 T fromBytes(T)(in ubyte[] bytes)
 {
      ByteConverter!T converter;
      converter.bytes = bytes;
      return converter.value;
 }

 void main(string[] args)
 {
      writeln(fromBytes!uint(toBytes(34u)));// should print 32 but prints
 1244564
 }

 why dose it print 1244564 the array is static size so why is it acting
 like bytes is a pointer not a static array?


Now it prints 34. :)

Ali
May 29 2015
prev sibling parent "Vladimir Panteleev" <vladimir thecybershadow.net> writes:
On Friday, 29 May 2015 at 07:55:55 UTC, seen wrote:

 ubyte[] toBytes(T)(T from) if (!is(T == const))
 {
 	ByteConverter!T converter;
 	converter.value = from;
 	return converter.bytes;
 }


This function is incorrect: it is returning a slice (dynamic 
array) of a static array, which is located on the stack. The fact 
that it compiles without error is bug 9279 
(https://issues.dlang.org/show_bug.cgi?id=9279).
May 29 2015