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digitalmars.D.learn - Can't use toHexString

reply "badlink" <andrea.9940 gmail.com> writes:
Hello,
I get different results if I either use toHexString or 
crcHexString.
I can't explain this behavior because crcHexString is just an 
alias of toHexString...

Test code: http://pastebin.com/33Ye7yyJ
Jul 19 2015
next sibling parent reply "Kagamin" <spam here.lot> writes:
http://forum.dlang.org/post/ydaeytbyxdnwlbpwkoma forum.dlang.org
Jul 19 2015
parent "badlink" <andrea.9940 gmail.com> writes:
On Sunday, 19 July 2015 at 12:00:23 UTC, Kagamin wrote:

 http://forum.dlang.org/post/ydaeytbyxdnwlbpwkoma forum.dlang.org


Argh... must use the search function first...

If I understand correctly in my case toHexString is returning a 
static char[len].
But why it gets silently converted to an immutable(char)[] ?
And why crcHexString works fine ? It's an alias to the same 
function...
Jul 19 2015
prev sibling parent reply "Adam D. Ruppe" <destructionator gmail.com> writes:
This line is illegal:


 return toHexString!(Order.decreasing)(crc.finish());


The std.digest.toHexString and variants return a *static array* 
which is static data and has a scope lifetime.

It is horrendous a compiler bug that allows it to implicitly 
convert to string - it should not compile because it silently 
gives bad results, escaping a reference to temporary memory while 
claiming it is immutable.

Confusingly though, this line is fine:


 return crcHexString(crc.finish());


It is an EXTREMELY subtle difference in the function signature 
(and totally bug prone, yet the documentation doesn't call it 
out...)




Notice the first overload there returns a static array, 
char[num*2], two of them return `auto`, so who knows what the 
hell they do.... and one actually returns `string`.

Slightly different arguments will give you wildly different 
results.



The safest thing to do is to `import std.conv;` and always 
`to!string` any of those *HexString return values if you want to 
return them like this.

Or you could also keep them in a local variable and use them 
before the function returns, then the stack allocated static 
array is OK too.
Jul 19 2015
next sibling parent "badlink" <andrea.9940 gmail.com> writes:
On Sunday, 19 July 2015 at 12:08:18 UTC, Adam D. Ruppe wrote:

 This line is illegal:


 return toHexString!(Order.decreasing)(crc.finish());


 The std.digest.toHexString and variants return a *static array* 
 which is static data and has a scope lifetime.

 It is horrendous a compiler bug that allows it to implicitly 
 convert to string - it should not compile because it silently 
 gives bad results, escaping a reference to temporary memory 
 while claiming it is immutable.

 Confusingly though, this line is fine:


 return crcHexString(crc.finish());


 It is an EXTREMELY subtle difference in the function signature 
 (and totally bug prone, yet the documentation doesn't call it 
 out...)
 ...


Thank you very much for the explanation.
Jul 19 2015
prev sibling parent Johannes Pfau <nospam example.com> writes:
Am Sun, 19 Jul 2015 12:08:16 +0000
schrieb "Adam D. Ruppe" <destructionator gmail.com>:


 This line is illegal:
 

 return toHexString!(Order.decreasing)(crc.finish());


 
 The std.digest.toHexString and variants return a *static array* 
 which is static data and has a scope lifetime.
 
 It is horrendous a compiler bug that allows it to implicitly 
 convert to string - it should not compile because it silently 
 gives bad results, escaping a reference to temporary memory while 
 claiming it is immutable.
 
 Confusingly though, this line is fine:
 

 return crcHexString(crc.finish());


 
 It is an EXTREMELY subtle difference in the function signature 
 (and totally bug prone, yet the documentation doesn't call it 
 out...)
 


Good catch. The version returning a string is meant to be used with the
interface digest API which produces ubyte[]. As we don't know the
length at compile time in that case we can't return a static array. But
in hindsight it might have been better to use another function for this
and not an overload, especially considering the static array=>string
conversion bug.

Documentation pull request:
https://github.com/D-Programming-Language/phobos/pull/3500

The good news is that the coversion bug has finally been fixed:
https://issues.dlang.org/show_bug.cgi?id=9279
Jul 19 2015