Hi, I've heard rumours that GCC 5 is coming :-)
I help maintain several C++ libraries and expect some work is required to get through this GCC transition. I'd like to understand what I'm doing and do it right the first time. I'm just an average C++ programmer, not an avid follower of GCC nor even of debian lists. So below are lots of questions asked out of ignorance. Please don't shoot the messenger/questioner. :-) Posts here and bug reports reference the wiki page https://wiki.debian.org/GCC5 . I've read that page and there are a bunch of notes embedded within it that just confuse me: 1. "The good news is, that GCC 5 now provides a stable libcxx11 ABI, and stable support for C++11 (GCC version before 5 called this supported experimental). This required some changes in the libstdc++ ABI, and now libstdc++6 provides a dual ABI, the classic libcxx98 ABI, and the new libcxx11 (FIXME: GCC 5 (<< 5.1.1-20) only provides the classic libcxx98 ABI)." What does the FIXME refer to? That the correct statement is "The good new is, that GCC 5 (post 5.1.1-20) now provides .."? Or something else? 2. "libstdc++ doesn't change the soname, provides a dual ABI. Existing C++98 binary packages will continue to work. Building these packages using g++-5 is expected to work after build failures are fixed. FIXME: Will they only work when built with _GLIBCXX_USE_CXX11_ABI set to 0? By default they will use the new libstdc++ ABI." What's the answer to this FIXME? What does the last sentence mean? Does it mean that code built using g++5 by default uses the new libstdc++ ABI? But if not rebuilt, existing code will use the existing libcxx98 ABI? The next paragraph says "GCC 6 is expected to change the c++ standard default ..." -- but doesn't GCC 5 change the standard default already? 3. "Library packages built using -std=c++0x or -std=c++11 may have an ABI change (unknown yet how many). How to find out about these?" What does this mean? Why would they have an ABI change? And how do we find out? 4. "If the library exports some symbols having either _cxx11 or B5cxx11 in the symbol name, it may be incompatible, if these are symbols which form part of the public API. To find out if these are part of the public API, you would need to build all reverse dependencies and see if any of these new symbols are referenced." Being pedantic: isn't that a signal of ABI (rather than API) change? More importantly: supposing my library does export some such symbols. Checking reverse deps in Debian is of course helpful: if any are referenced, then we know the interface changed (or does it? what if the reverse-dep uses the symbol itself internally -- might this show a false-positive?) But if none are found, can we conclude the interface has NOT changed? I wouldn't think so: it may simply be that the code in Debian simply doesn't exercise the relevant part of the interface. If that is the case: how then can one convince themself that there really is no change in interface? Additionally, there are some places where the text confuses me even without FIXME or README: * "Using different libstdc++ ABIs in the same object or in the same library is allowed, as long as you don't try to pass std::list to something expecting std::cxx11::list or vice versa. We should rebuild everything with g++-5 (once it is the default). Using g++-4.9 as a fallback won't be possible in many cases." The first sentence doesn't seem connected to the second two sentences. What does "Using g++-4.9 as a fallback ..." mean? Why isn't it possible? What cases fail? * In "Roadmap for libstdc++", it says "Depending on which libstdc++ ABI is configured as default ...". What is the plan now? Which ABI will be default? * Section "libstdc++ c++11 incompatibilities (4.9 and 5)" has a short list of incompatibilities. Is this the complete list or just examples? * "Passing std::list to something expecting std::cxx11::list or vice versa." Is this an incompatibility just regarding C++11 in 4.9 and 5? What if C++11 code (built with GCC5) passes a list to an old library built using the libcxx98 ABI? Wouldn't that also fail? Thanks for reading this far! -Steve
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