Re: [boost] null

From: Martin Wille (mw8329_at_[hidden])
Date: 2005-07-29 01:52:43

Jason Hise wrote:
> Martin Wille wrote:
> 
> 
>>Jason Hise wrote:
[...]
>>I also object to this idea in general. It doesn't buy us much, it breaks 
>>existing code, it may cause ODR violations, and it also violates the C++ 
>>Standard.
>>
> 
> What existing code will it break?  I would imagine it would only break 
> code that deserves to be broken in the first place (like int i = NULL).

Eric already provided an example for code breaking with your definition 
of NULL.

There's nothing wrong with int i = NULL. It's certainly not good style, 
but that doesn't make it deserve to be broken.

> Also, how would this cause ODR violations?  I was under the impression 
> that the one definition rule required all definitions in different 
> translation units to be identical, not that it required that a 
> definition only be provided in one translation unit.

a.hpp:
#include "boost/null.hpp"

b.hpp:
#include <cstddef>

c.hpp:
/* e.g. template code that uses NULL */

x.cpp:

#include "b.hpp"
#include "a.hpp"
#include "c.hpp"

/* use stuff from c.hpp */

y.hpp:

#include "b.hpp"
#include "c.hpp"

/* use stuff from c.hpp */

x and y will instantiate different code due to the different #include order.

   And even if
> somehow more than one instance of null ended up being used, how would 
> this actually end up causing a problem?

It's undefined behaviour.

                                         Would your problem with this
> code vanish if I removed the NULL definition on the last two lines?

Yes.

> Would it reappear if I moved the null class into the global namespace 
> (essentially making lowercase null a keyword)?

Adding stuff to the global namespace bears its own set of problems.
You would definitely break a lot of code that defines its own null 
constants. It's certainly less of a problem than re#defining NULL would be.

Regards,
m
Send instant messages to your online friends http://au.messenger.yahoo.com