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

From: Daryle Walker (darylew_at_[hidden])
Date: 2008-07-18 05:07:08

On Jul 17, 2008, at 10:32 PM, David Abrahams wrote:

> on Thu Jul 17 2008, Daryle Walker <darylew-AT-hotmail.com> wrote:
>>
[SNIP]
>> Let's define some stuff here:
>> T: some type you created, probably a class type
>> a: an object of type T; it's valid, i.e. meets its invariants
>> b: another object of type T, also valid; any similarities or
>> differences in its state from a's state is unspecified
>> C: the set of all valid states an object of type T may have
>>
>> Let function F: x -> Y, map a T object state x to a (sub)set of C
>> named Y.  This function returns the subset of source states that a
>> given state can receive during an assignment.
>>
>> If there exists at least one x in C such that F(x) = Y < C, then type
>> T is _NOT_ Assignable!  In other words, the number of assignment-
>> compatibility classes in T must be exactly one for T to be  
>> Assignable.
>
> 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.

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