libc++ ABI versioning

I've noticed that Xcode 4.6 and Xcode 5.1 ship with headers that have
different layouts for std::ifstream. sizeof(std::ifstream) is 448 for
4.6, and 576 on 5.1. You can see my test case here:
https://gist.github.com/anonymous/bd0db5e4bcb307b70506

How does clang/libc++ ensure that the executable runs with the right
libc++ version for headers it was built with? I can run the binaries
produced by Xcode 4.6 on a machine that has a newer libc++, and I can
run the binaries produced by 5.1 on a machine that has an older
libc++. I would assume that if I built with headers that give me a
smaller size than what is expected by libc++, that would be cause the
newer libc++ to trash memory beyond the end of my object. Am I missing
something?

I was unable to find any resources on how libc++ ABI versioning works
other than the "major ABI version" -- which is 1.

The specific problem I'm seeing is that it seems that when I link with
a static library that was built with the older libc++ version, a
translation unit built with the new libc++ ends up using the wrong
(old) value for the construction symbol at runtime
(__ZTCNSt3__114basic_ifstreamIcNS_11char_traitsIcEEEE0_NS_13basic_istreamIcS2_EE,
or '
   construction vtable for std::__1::basic_istream<char,
std::__1::char_traits<char> >-in-std::__1::basic_ifstream<char,
std::__1::char_traits<char> >' according to c++filt).

- Jørgen.

Looks like the difference is probably due to the std::filebuf subobject.
Did that type also change size? Did std::basic_streambuf<char>?

Yep, filebuf did, and that does seem to account for the difference in size.

themac:~ jorgen$ ./foo-modern
sizeof(std::ifstream)=576, offset=0
sizeof(std::istream)=168, offset=0
sizeof(std::ios)=152, offset=424
sizeof(std::ios_base)=136, offset=424
sizeof(std::filebuf)=408, offset=0
sizeof(std::basic_streambuf<char>)=64, offset=0

themac:~ jorgen$ ./foo
sizeof(std::ifstream)=448, offset=0
sizeof(std::istream)=168, offset=0
sizeof(std::ios)=152, offset=296
sizeof(std::ios_base)=136, offset=296
sizeof(std::filebuf)=280, offset=0
sizeof(std::basic_streambuf<char>)=64, offset=0

- Jørgen.

Looks like this commit was the culprit:

r162601 | hhinnant | 2012-08-24 14:20:56 -0700 (Fri, 24 Aug 2012) | 31 lines

It added a new member variable
  state_type __st_last_;

to the basic_filebuf object, and that variable is 128 bytes long.

— Marshall

So why is this not an ABI-breaking change?

What causes a program built against the earlier version of
std::ifstream to correctly execute on a machine that has the latest
version of std::ifstream that expects a larger layout?

- Jørgen.

I think it was an ABI-breaking change.
I also believe that it was done w/o realizing that it was breaking the ABI.

— Marshall

So does this mean I need to be linking statically with libc++? :confused:

- Jørgen.