Please explain the C++ ABI

Question

The common explanation for not fixing some issues with C++ is that it would break the ABI and require recompilation, but on the other hand I encounter statements like this:

Honestly, this is true for pretty much all C++ non-POD types, not just exceptions. It is possible to use C++ objects across library boundaries but generally only so long as all of the code is compiled and linked using the same tools and standard libraries. This is why, for example, there are boost binaries for all of the major versions of MSVC.

(from this SO answer)

So does C++ have a stable ABI or not?

If it does, can I mix and match executables and libraries compiled with different toolsets on the same platform (for example VC++ and GCC on Windows)? And if it does not, is there any way to do that?

And more importantly, if there is no stable ABI in C++, why are people so concerned about breaking it?

Answer 1

Although the C++ Standard doesn't prescribe any ABI, some actual implementations try hard to preserve ABI compatibility between versions of the toolchain. E.g. with GCC 4.x, it was possible to use a library linked against an older version of libstdc++, from a program that's compiled by a newer toolchain with a newer libstdc++. The older versions of the symbols expected by the library are provided by the newer libstdc++.so, and layouts of the classes defined in the C++ Standard Library are the same.

But when C++11 introduced the new requirements to std::string and std::list, these couldn't be implemented in libstdc++ without changing the layout of these classes. This means that, if you don't use the _GLIBCXX_USE_CXX11_ABI=0 kludge with GCC 5 and higher, you can't pass e.g. std::string objects between a GCC4-compiled library and a GCC5-compiled program. So the ABI was broken.

Some C++ implementations don't try that hard to have compatible ABI: e.g. MSVC++ doesn't provide such compatibility between major compiler releases (see this question), so one has to provide different versions of library to use with different versions of MSVC++.

So, in general, you can't mix and match libraries and executables compiled with different versions even of the same toolchain.