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Subject: Re: [boost] [config] Macro for null pointer
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2012-11-29 13:40:26

Le 29/11/12 08:23, Jeffrey Lee Hellrung, Jr. a écrit :
> On Wed, Nov 28, 2012 at 11:15 PM, Andrey Semashev <andrey.semashev_at_[hidden]
>> wrote:
>> On Wed, Nov 28, 2012 at 8:43 PM, Jeffrey Lee Hellrung, Jr.
>> <jeffrey.hellrung_at_[hidden]> wrote:
>>>> using boost::nullptr;
>>> One thing I wonder about is if this using declaration is The Right Thing
>> To
>>> Do. An alternative is to provide a macro to bring the nullptr identifier
>>> into the current scope (and which does nothing in C++11).
>> I don't like the macro with using declaration. I'd rather have this
>> using declaration in the header, with possibility to disable it by
>> defining a config macro:
>> using boost::nullptr;
>> #endif
>> And I want this declaration to be enabled by default since the
>> language keyword is not scoped in a namespace and that's what we try
>> to emulate. The macro switch is only there to solve problems if they
>> arise.
> Eh I don't think it's a good idea to control scope via a configuration
> macro. Anything that uses it unqualified (and which you have no control
> over) would suddenly break upon disabling the using declaration.

How the using declaration would help if a 3pp library provides already
nullptr in the global namespace? This will be ambiguous, isn't it?
> In bringing this up, I'm thinking of the case in which some other 3rd party
> decided that they, too, wanted to provide a nullptr emulation, so now we
> have 2 conflicting nullptr identifiers at global scope. That makes Boost
> and this 3rd party library unusable together.
The macro to disable the using declaration makes them usable together,
but the problem is that now code that was using the bool emulation will
use the 3pp one.

I don't think there is a global solution to this problem. I think that
the library should provide the using declaration by default so a user
that knows that his application would not have any 3pp trouble could use
the unqualified nullptr. Unfortunately libraries authors including Boost
libraries should use nullptr qualified as ::boost::nullptr to avoid any
possible conflicts.


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