Re: [boost] [HEAD] lexical_cast<int8_t>("127") bug?

From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2007-05-09 15:40:56

Jens wrote:
> IMHO what this discussion shows is that in this case there's no "obvious" meaning of 8 bit 
> integer types (uses seem to be number, character or byte). As far as I am concerned, that 
> implies that it's up to the user to supply that meaning, and not for the library to assign 
> one.

Yes.  But there are two existing ways for the user to say what they mean:

(1) By using int8_t / uint8_t for number-bytes and char for 
character-bytes.  For this to work we would need to either
   (a) Do some magic so we can distinguish [un]signed char from 
[u]int8_t.  I believe that's impossible.
   (a) Change the behaviour of standard streams for [un]signed char.  
That's not going to happen, and even if it could happen it would 
probably break significant amounts of code that uses 'unsigned char' 
for characters.
   (b) Change lexical_cast to special case 'unsigned char' and 'signed 
char'.  This is clearly unpopular with the list though I personally 
would be happy with it.
   (c) Allow the user to enable (b) if that is what they want.  I get 
the impression that this could be done by supplying your own partial 
specialisation of lexical_cast; is this true?  (Off topic, there is the 
question of whether common lexical casts can be made more efficient by 
providing specialisations that invoke C library functions like 
strtol(); I think someone told me that this wasn't possible, but I 
don't think I ever knew why.)

(2) By using format() instead of lexical_cast().  (For to-string 
conversions.  In principle something symmetric could be provided for 
from-string conversions.)  Using format, you supply a format specifier 
in the style of printf() that indicates whether you want to do a string 
or numeric conversion.  However, *this format specifier is ignored* and 
if you ask for a '%d' int8_t you'll still get a character.

I would much prefer to have one of these existing methods 'do what I 
want', rather than invent something else.

I'm going to see if I can write a my_lexical_cast() that uses 
lexical_cast by default but changes the behaviour for [un]signed char.

Phil.