Martin Sebor wrote: > >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.
I don't either. >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. Well, perhaps we should disable _RWSTD_EXT_CXX_0X for compilers for which we define _RWSTD_NO_RVALUE_REFERENCES and _RWSTD_NO_VARIADIC_TEMPLATES. Then we could remove any workarounds for compilers that don't support these features. That reduces the amount of workaround code that has to be created and maintained. > >> 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 >> > >
