Re: [boost] C++03 / C++11 compatibility question for compiled libraries

Subject: Re: [boost] C++03 / C++11 compatibility question for compiled libraries
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2018-02-10 17:25:53

On 02/10/18 20:13, Peter Dimov via Boost wrote:
> Andrey Semashev wrote:
>> On 02/10/18 18:43, Peter Dimov via Boost wrote:
>> >
>> > But, if you really wanted to know, libstdc++ in a situation like the 
>> > above (when the function has to be virtual because it's f.ex. 
>> specified > that way by the standard) does something like the following:
>> >
>> > #if cpp11
>> >
>> > virtual int __f( boost::function<int ()> const& F ) = 0;
>> > virtual int f( std::function<int ()> const& F ) = 0;
>> >
>> > #else
>> >
>> > virtual int f( boost::function<int ()> const& F ) = 0;
>> > virtual int __f( std::function<int ()> const& F ) = 0;
>> >
>> > #endif
>> >
>> > except of course we don't have std::function in C++03 mode, so even 
>> > declaring __f will be a problem.
>>
>> If you rely on the particular algorithm of vftable slot allocation,
> 
> I've never heard of a vtable slot allocation algorithm that is not the 
> particular one that everyone uses.
> 
>> declaring is not a problem - you can declare pretty much whatever 
>> virtual function. It's not going to be called anyway and its sole 
>> purpose is to reserve a particular slot.
> 
> It is going to be called, when a C++11 exe calls a C++03 library.

Well, yes, but that's not a problem for libstdc++, because, presumably, 
it is always compiled with the latest C++ supported by the compiler. So 
the std::function declaration is only replaced with a dummy when it 
cannot be called.

Maybe a similar convention is possible for Boost as well, i.e. we 
support linking executables compiled only for C++ <= N, where N is the 
version that Boost was compiled for. It's difficult to enforce, though.