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

From: Daryle Walker (darylew_at_[hidden])
Date: 2008-07-18 06:48:03

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.  This means that your program is illegitimate and we can't  
count it as a counter-example.

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