Re: [RFC] C++1x breaking the ABI in one more place :(

[Jason Merrill <jason at redhat dot com>]

On 05/19/2010 06:26 PM, Paolo Carlini wrote:
> 80    std::time_get<_CharT, _InIter>::do_get [with _CharT = char,
> _InIter = std::istreambuf_iterator<char, std::char_traits<char>  >,
> iter_type = std::istreambuf_iterator<char, std::char_traits<char>  >,
> std::ios_base::iostate = std::_Ios_Iostate]
>
> Now I need help about figuring out of serious is this issue: suppose we
> are exporting this new vtable and an old binary tries to link against
> it. Would it break?

As I mentioned before, I believe it will only break if something in the 
binary both derives from time_get and adds additional virtual functions, 
e.g.

class mytime : std::time_get<...>
{
  virtual void dostuff ();
}

If this were compiled against the old time_get, it would put dostuff() 
at position 80 in the vtable that mytime shares with time_get; compiled 
against the new time_get dostuff() would be at offset 88, which breaks 
binary compatibility.

I don't know how likely this scenario is.

Jason