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dave_at_[hidden]

--
Dave Abrahams
Boostpro Computing
http://boostpro.com
On Jul 18, 2008, at 7:30 AM, Daryle Walker <darylew_at_[hidden]> wrote:
> On Jul 18, 2008, at 6:57 AM, David Abrahams wrote:
>
>> on Fri Jul 18 2008, Daryle Walker <darylew-AT-hotmail.com> wrote:
>>
>>> On Jul 18, 2008, at 5:07 AM, Daryle Walker wrote:
>>>
>>>> On Jul 17, 2008, at 10:32 PM, David Abrahams wrote:
>>>>>
>>> [SNIP]
>>>>> Then, if I understand you correctly, none of the built-in types  
>>>>> are
>>>>> Assignable.
>>>>>
>>>>>        char* p;     // p is unintialized
>>>>>        char* q = p; // invalid
>>>>>
>>>>> Yes, uninitialized is one of the valid states for a builtin type,
>>>>> i.e. part of the type's invariants.
>>>>
>>>> Really, I was wondering about that (corner) case, especially since
>>>> it can't be replicated (i.e. it's undefined to use such a state as
>>>> a source).  I'm thinking more about non-POD class types, which must
>>>> have an initial state with the internal primitive objects
>>>> initialized.
>>>
>>>
>>> Well, I looked into it further.  In C++ 2003, section 4.1 "Lvalue- 
>>> to-
>>> rvalue conversion" [conv.lval], paragraph 1, an uninitialized object
>>> can only be used as an lvalue, converting it to a rvalue is  
>>> undefined
>>> behavior.
>>
>> Yes, that's what "// invalid" means.
>>
>>> This means that your program is illegitimate and we can't count it  
>>> as
>>> a counter-example.
>>
>> Huh?  By that logic no counterexample is possible.  Or am I missing
>> something?
>
> Yes, the OP was having a problem with a boost class (template) that  
> is like std::valarray: you can only do assignments if the source and  
> destination objects _already_ had the same layout.
I understand that
> And my first response to this thread explained why this is  
> problematic for our users,
I understand that too. My problem is with your implicit declaration  
that such types are not to be considered Assignable.  That concept is  
supposed to be compatible with the builtins.
> and so we shouldn't do this.  In this setup, all objects are validly  
> initialized; it's just there isn't a single assignment-compatibility  
> class that all valid object states belong to.  Note that you have to  
> go out of your way to create a class like this.
Not really. It's pretty easy to end up with uninitialized members. But  
that's really beside the point.
>  It's a pain because the user who wants to use this class internally  
> has either make sure all wrapping objects keep all sub-objects of  
> this type within the same assignment-compatibility class or write a  
> custom assignment routine.  Said routine can be at most the basic  
> guarantee if the resizing or copy steps may throw.
So?  I don't see how that's related or why it's a problem
>  (A strong guarantee could be done if the type provides a swap that  
> can cross assignment-compatibility classes.)  An author-supplied  
> full-assignment routine could take advantage of the internal  
> implementation and add rollback.