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From Martin Sebor <>
Subject Re: ABI stability of aligned_union et al
Date Thu, 26 Jun 2008 23:00:25 GMT
Martin Sebor wrote:
> The mangling of std::aligned_union depends on support for variadic
> templates in that specializations of the template will be mangled
> differently depending on whether _RWSTD_NO_VARIADIC_TEMPLATES is
> #defined or not. It seems that it should be possible to assure
> compatible mangling between the two,

After thinking about it some more I no longer believe this is doable.
With variadic templates the mangled aligned_union will involve only
the names of the types the template is instantiated on. Without them
I don't see any way to emulate this unique behavior. I.e., given
a class template X that takes, say, 2 template arguments, I don't
see a way to prevent the compiler from including the names of both
template arguments in the mangled name for the specialization of
the template on just one argument. The second argument is always
there, even if it's not actually referenced in code.

So it seems to me that if we provide "workarounds" for the absence
of support for variadic templates in the compiler there will be no
way to migrate from a compiler without the feature to one with it
without breaking the ABI of code that uses templates like
aligned_union or tuple.

> but I'm not 100% sure, and
> if it is, that it's worth the effort. There might be other
> templates where we may not easily be able to achieve compatibility.
> I suppose the big question then is: should we try to fortify the
> implementation against ABI changes caused by changes in the
> techniques used in the implementation of C++ 0x? In the past we
> didn't concern ourselves with this issue but since we can expect
> compilers to only start supporting some of the new core language
> features such as variadic templates only slowly and piecemeal,
> a few at a time, I think it's a concern that we need to address
> early on.
> Thoughts?
> Martin

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