Re: [boost] [config] Variadic template macros in gcc header

Subject: Re: [boost] [config] Variadic template macros in gcc header
From: Edward Diener (eldiener_at_[hidden])
Date: 2010-11-22 18:26:38

On 11/22/2010 2:40 PM, Daniel James wrote:
> On 22 November 2010 14:33, Edward Diener<eldiener_at_[hidden]>  wrote:
>>
>> Is this also the intention of Boost in general, that an empty config.h means
>> a compiler that supports C++03 ?
>
> In general, an empty config.hpp means C++0x. That did seem to be the
> consensus decision, and I think everyone went into it understanding
> the consequences. It looks like the decision was made here (only the
> first few replies are relevant):
>
> http://thread.gmane.org/gmane.comp.lib.boost.devel/175638

In that case Peter Dimov's assumption seems wrong. He is saying that he 
did not want you to make the changes because an empty config.h should 
mean the C++ standard, aka C++03.

>
> I also found some earlier discussion where I think the change was
> first proposed (only the first subthread):
>
> http://thread.gmane.org/gmane.comp.lib.boost.devel/151193
>
> Interestingly, someone proposed that the C++0x BOOST_NO_ macros should
> be defined by default, and then undefined in the config headers.
>
> Personally, I don't have a strong opinion and will follow the consensus.

My persoanl opinion is:

1) It is confusing to have two different ways to define the lack or 
presence of some C++ feature in Boost. I really think this is a bad 
approach. It not only becomes a maintenance headache, since any Boost 
library can use either of the two ways, but it is also confusing to new 
library developers using or not using C++ features in Boost. I feel very 
strongly that Boost should strive for a single configuration macro which 
means a specific thing.

2) If no config.h means C++0x and not C++03 then you should be able to 
make the change you had intended to do despite Peter's objection to it. 
BTW my search shows now a few other instances where your change to get 
rid of the BOOST_HAS_ versions regarding variadic templates and rvalue 
references would need to be made if you revisit the change.

3)

a) I can easily live with the BOOST_NO_ macros defined by default and 
then #undef'ed in the appropriate cases if that solves a usage problem, 
but others might find that backward. Even in that case config.h should 
not be empty but would need to at least include whatever common header 
file defined all the BOOST_NO_ macros by default. But I think the 
current solution is fine as long as everyone agrees that a truly empty 
config.h means a C++0x compliant compiler, as rare as that may actually 
be currently.

b) If on the other hand, because of the rarity of a C++0x compiler in 
terms of Boost features, it is felt that an empty config.h really should 
mean just current C++ standard compliance, aka C++03, then the system of 
the way that BOOST_NO_ macros are defined seems wrong to me and at least 
the BOOST_NO_ macros which refer strictly to C++0x features should be 
defined by default, undef'ed as needed, and even an empty config.h must 
include those definitions, very probably by including a header file 
which makes them. This change would of course entail many changes in the 
config header files regarding these macros.

Finally I am not arguing for or against what Peter objected to. Based on 
his assumption he was perfectly correct.