[Interest] [Mingw-w64-public] Compiling Qt (4 or 5) with "-std=c++11"

[Thiago Macieira]

On quinta-feira, 21 de março de 2013 17.30.12, K. Frank wrote:
> Hi Thiago!
> 
> (I've taken the liberty of cross-posting this back to the mingw-w64-public
> list.)

Sure, no problem. I'll keep them on CC, even though I don't think my reply is 
relevant to them.

> > Qt automatically switches to that mode in the modules that require it.
> > 
> >> As I understand it, using "-std=c++11" causes abi breakage, so to do
> >> this, I will have to recompile the various libraries I use.
> > 
> > Not with Qt. Qt has the very same ABI, whether you compile it with C++11
> > or
> > C++98.
> 
> I guess I should take your word for it.  But I'm a little confused, so let
> me ask for some clarification.  How does Qt control the abi produced by the
> compiler?  I was under the distinct impression that "-std=???11" caused
> significant abi breakage, that this was recognized as an issue, but, I
> guess, that the gcc folks felt that it was worth it for some reason.
> 
> How could Qt manage to dodge that bullet?  Or am I misunderstanding
> the issue?

You're misunderstanding the issue.

The compiler ABI is exactly the same. There has been no change to the calling 
convention or the naming of external symbols ("mangling") for existing 
functions. The mangling has new codes so it can support the new C++11 types 
and features (Ds and Di for char16_t and char32_t and support for variadic 
template packs, for example).

However, Qt is quite clever about the use of such features: those types and 
features only appear in inline functions. Therefore, the external symbols 
exported by Qt are the same, no matter how you compile the Qt libraries.

That's not actually the case if you use Visual Studio, but there the situation 
is completely different. For starters, you can't choose to enable C++11: it's 
always enabled. And second, Microsoft hasn't heard yet about C++ binary 
compatibility. If you upgrade your compiler, you must recompile everything, so 
it doesn't matter which features we use.

You may be actually thinking of some reports that the Standard C++ Library 
that comes with GCC broke compatibility. I've heard the same, and I've also 
heard that it was reverted, or misunderstood. So in the end, I simply don't 
know what the situation is. But it's irrelevant for Qt, since we do not use 
the Standard Library in our public, non-inline functions.

If you're interested in a binary compatibility topic that might affect you -- 
and since you so kindly brought the mingw community in -- be careful about the 
C++ exception model from your mingw compiler. They are binary-incompatible 
with one another, if you use exceptions. The MinGW community needs to choose 
one model, once and for all, deprecate all others, period. Just choose one, 
any one, provided it's not SJLJ. If your compiler uses SJLJ, run from it and 
avoid it like the plague. If that's the only option available, choose another 
compiler.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
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