Re: [Boost-users] [boost] [multi_array] Everyone, please make your value-types Assignable!

From: Daryle Walker (darylew_at_[hidden])
Date: 2008-07-18 07:30:42

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.  And my first  
response to this thread explained why this is problematic for our  
users, 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.   
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.  (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.

-- 
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT hotmail DOT com