Do Visual Studio 2012 updates break C++ ABI?

Question

When Microsoft initially released Visual Studio 2012 in September 2012, they announced their plan for providing updates for Visual Studio on a more regular basis. Since then, they have released Visual Studio 2012 Update 1 (Visual Studio 2012.1) in November 2012 and Visual Studio 2012 Update 2 (Visual Studio 2012.2) in April 2013.

My question is: Did the updates introduce any changes to the C++ ABI (with regard to the initial VS2012 version)? Is it safe to link .libs of different VS2012 versions?

I have searched the internet for a while and could not find any definite statement from Microsoft. Some sources mention that some bugs in the C++ code generation have been fixed but I suppose that does not imply an ABI change?

Answer 1

Stephan T. Lavavej, a key author of Visual C++'s STL implementation laid out the rules in this Reddit thread:

Here are the precise rules:

If you include any C++ Standard Library headers, you have to play by its rules, and we intentionally break binary compatibility between major versions (but preserve it between hotfixes and service packs). Any representation changes (including but not limited to adding/removing data members) break binary compatibility, which is why this always happens and why we jealously guard this right.

[snip]

So, if you're playing by the STL's rules, you need to ensure the following:

• All object files and static libraries linked into a single binary (EXE/DLL) must be compiled with the same major version. We added linker checks, so that mismatching VS 2010+ major versions will trigger hard errors at link time, but if VS 2008 or earlier is involved we can't help you (no time machines). Because the ODR applies here, you really should be using the same toolset (i.e. same service pack level) for all of the object files and static libraries. For example, we fixed a std::string memory leak between VS 2010 RTM and SP1, but if you mix RTM and SP1, the resulting binary may or may not be affected by the leak. (Additionally, you need to be using the same _ITERATOR_DEBUG_LEVEL and release/debug settings; we have linker checks for these now.)
• If you have multiple binaries loaded into the same process, and they pass C++ Standard Library objects to each other, those binaries must be built with the same major version and _ITERATOR_DEBUG_LEVEL settings (release/debug should match too, I forget if you can get away with mismatch here). Importantly, we cannot detect violations of this rule, so it's up to you to follow it.
• Multiple binaries whose interfaces are purely C or COM (or now WinRT) may internally use different major versions, because those things guarantee binary compatibility. If your interfaces involve the C++ Core Language (e.g. stuff with virtuals) but are extremely careful to never mention any C++ Standard Library types, then you are probably okay - the compiler really tries to avoid breaking binary compatibility.

Note, however, that when multiple binaries loaded into a single process are compiled with different major versions, you'll almost certainly end up with multiple CRTs loaded into your process, which is undesirable.

Bottom line - if you compile everything 100% consistently, you just don't have to worry about any of this stuff. Don't play mixing games if you can possibly avoid it.

Answer 2

Finally, I found an answer to my question in Stephan T. Lavavej's blog post C++11/14 STL Features, Fixes, And Breaking Changes In VS 2013:

The VS Update mechanism is primarily for shipping high-priority bugfixes, not for shipping new features, especially massive rewrites with breaking changes (which are tied to equally massive compiler changes).

Major versions like Visual C++ 2013 give us the freedom to change and break lots of stuff. There's simply no way we can ship this stuff in an Update.

Q5: What about the bugfixes? Can we get those in an Update?

A5: This is an interesting question because the answer depends on my choices (whereas in the previous question, I wouldn't be allowed to ship such a rewrite in an Update even if I wanted to).

Each team gets to choose which bugfixes they take to "shiproom" for consideration to be included in an Update. There are things shiproom won't let us get away with (e.g. binary breaking changes are forbidden outside of major versions), but otherwise we're given latitude to decide things. I personally prioritize bandwidth over latency - that is, I prefer to ship a greater total number of bugfixes in every major version, instead of shipping a lesser total number of bugfixes (over the same period of time) more frequently in multiple Updates.