Re: [Boost-users] [serialization]

From: Nigel Rantor (wiggly_at_[hidden])
Date: 2005-11-10 12:03:57

David Abrahams wrote:
> Nigel Rantor <wiggly_at_[hidden]> writes:
> 

Hi David,

Well, I don't have access to the raw spec, I am working from "The C++
Language 3rd Ed." so if I make some errors due to this you'll have to
forgive me.

>> Robert Ramey wrote:
>> 
>>> Paul Giaccone wrote:
>>> 
>>> 
>>>> For booleans, though, a value of other than 0 or 1 means it has
>>>> not been initialised, and perhaps this should throw an
>>>> exception on writing to the archive rather than on reading from
>>>> it.
>>> 
>>> 
>>> Hmmm, I'm not sure about this.  Do we know for a fact that a bool
>>>  variable will always contain 1 or 0?  I've never seen code trap
>>> on an un-initialized bool.  It seems that even an uninitialized
>>> bool corresponds to true or false.
>> 
>> No, bools won't always contain 1 or 0, like other types their value
>> is be undefined if they have not been initialised, depending upon
>> where they have been declared.
> 
> 
> An uninitialized bool doesn't have a value you can read without 
> causing undefined behavior.  So for all intents and purposes it 
> doesn't have a value.

My understanding of uninitialised variables is that their *values* were 
undefined, that you could not rely on them to be any particular value, 
including not being within range for that type.

So, you can read them, but there are no guarantees about what you'll get 
back. I suppose I agree, it really isn't a bool yet unless you can be 
assured that it contains true or false, but it does have a value.

>> <caveat> C++ isn't my day job...I just use it for fun things... 
>> </caveat>
>> 
>> When bools are used in logical operations they are converted to 
>> integers
> 
> 
> Can you cite the standard on that one?  I'm pretty sure that it's the
>  other way around: when integers are used in logical operations they 
> are converted to bools.

No, but I read it in the fourth para of sec 4.2 of TCPPPL.

"In arithmetic and logical expressions, bools are converted to ints; 
integer arithmetic and logical operations are performed on the converted 
values."

>> , so depending on what your bool happens to contain before 
>> initialisation it could evaluate to either true or false.
> 
> Or it could crash your computer.

Computer or program? Really? Please elaborate, I'm interested. I don't 
see how accessing a piece of memory that hasn't been initialised to a 
legal value would cause that.

>>> Perhaps part of the problem is that I used 0 and 1 for bool
>>> variable in order to not included english strings "true" and
>>> "false" in text files and to save space.
>>> 
>>> I'll think about this.
>> 
>> My two proposals would be the one that encourages pragmatism and
>> the other that encourages correctness.
>> 
>> Pragmatic:
>> 
>> Well, how about simply treating anything other than 1 as false?
> 
> 
> I don't know what 1 has to do with anything.  The values of a bool 
> are true and false.  1 is an int.

I was simply using the OP's terminology, I apologise. s/1/true/ in my 
above sentence. :-)

Anyway, the two approaches that I described for this problem still 
stand. Tell people to do it right or be lenient when parsing.

If Robert can find a nice way of trapping uninitialised variables (not 
simply bools of course) and throw exceptions for these then that's 
great. I'm not sure that is entirely possible though. (and I haven't 
psent any time thinking about it either)

   n