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

From: Jason Merrill <jason at redhat dot com>
To: Paolo Carlini <paolo dot carlini at oracle dot com>
Cc: libstdc++ <libstdc++ at gcc dot gnu dot org>, Ian Lance Taylor <iant at google dot com>, Jakub Jelinek <jakub at redhat dot com>
Date: Wed, 19 May 2010 14:27:13 -0400
Subject: Re: [RFC] C++1x breaking the ABI in one more place :(
References: <4BF42B5D.2000405@oracle.com>

On 05/19/2010 02:18 PM, Paolo Carlini wrote:

so I was finally getting around to adding the new C++1x member functions

It's still C++0x until we have a final number.

to std::time_get (also needed in order to implement the std::get_time
free function) when I noticed that we should also add a new *virtual*
function, do_get, and that breaks of course the ABI because, eg, on
x86_64, the size of the object itself grows from 80 to 88...

Off the top of my head, I can't imagine any trick we can play to avoid
breaking the ABI when adding an additional virtual, but I admit not
being such an expert in this area, and I'm looking for advice...

It already has virtual functions, so it shouldn't affect the size of the object. Adding an additional function to the end of the vtable should be backward binary compatible except for derived classes that add virtual functions (and therefore append them to the vtable, clashing with do_get).

I would bring this up with the library WG, though.

Jason

Follow-Ups:
- Re: [RFC] C++1x breaking the ABI in one more place :(
  - From: Paolo Carlini

References:
- [RFC] C++1x breaking the ABI in one more place :(
  - From: Paolo Carlini

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