Re: [Boost-build] switching on toolset

Subject: Re: [Boost-build] switching on toolset
From: Robert Ramey (ramey_at_[hidden])
Date: 2015-06-17 11:25:37

On 6/16/15 9:01 PM, Gennadiy Rozental wrote:
> Edward Diener <eldiener <at> tropicsoft.com> writes:
>
>> Look at predef-check and predef-require in the Boost predef library. It
>> is still a bit buggy but it is what you want.
>
> I do not understand how to use these or what they are doing.
>
> For now , what I need is rather simple. If toolset is gcc with version less
> then equal to 4.6 add std=c++0x otherwise add std=c++11.

would this really be a great idea?

And what happens when one adds the std=c++11/... to the bjam build 
globally?  and this conflicts with your choice here?

What happens when I build tests using std=c++11 and your library is 
using something else.

What happens when the user want's to build the library using CMake or 
some other build system.

This would mean that the functioning of the library might change 
silently dependending upon which compiler the user was using.  This kind 
of behavior leads to bugs and behavior which is maddenly difficult to 
understand.

How about doing the following:

a) Use the macros in config.hpp to condition code on availability of 
C++11/C++14 features.  These macros are dependent upon the std=c?? set 
by the user at the top level of bjam.

b) let the user apply this switch at the top level with the toolset 
attribute and let all the libraries follow this setting.  This will 
guarentee a consistent/unambiguous setup.

If it were up to me, bjam would give and error/warning if some lower 
level attribute setting conflicted with a previously set attribute 
value.  This would prevent these kinds of problems.  It would also make 
what you want to do impossible to do - which in my view would be a good 
thing.

Robert Ramey