Re: [boost] [config] Macro for null pointer

Subject: Re: [boost] [config] Macro for null pointer
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2012-11-29 02:32:46

On Thu, Nov 29, 2012 at 11:23 AM, Jeffrey Lee Hellrung, Jr.
<jeffrey.hellrung_at_[hidden]> wrote:
> 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:
>>
>> #ifndef BOOST_NO_GLOBAL_NULLPTR
>> 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.

You mean a third party library that relies on Boost emulation of
nullptr? I think, the library has every mean to mitigate this problem
(e.g. to check that nullptr is emulated and import it into its own
namespace).

There is no such problem with user's code as long as multiple
libraries try to emulate global nullptr. If that happens you just
disable all emulations but one. nullptr is still there, nothing
breaks.

> 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.

That's exactly the reason the config macro is provided. Until such
conflict appears one can safely use nullptr without qualification.