Il 30/12/2016 14:03, Philippe ha scritto: > What is the logic of the C++ standard to introduce a new keyword (alignas) > and have default containers ignoring them?...
There is no good reason, that's why this has been changed in C++17. The language and the standard library evolve and these mistakes are fixed. The hard part is how to fix them in a way that does not break the billions of lines of code out there, which already rely on the current, documented, standardized semantics for custom allocators / operator new. Did you notice that "new OverlyAlignedType" in pre-C++17 may fail to honour the alignment? (N3337 §5.3.4/1 [expr.new]: "It is implementation-defined whether over-oligned types are supported") > From my POV, QVector succeeds where the standard std::vector fails. Fails how...? > Yes, using a custom allocator is possible. > But this another case where Qt succeeds in its philosohphy of easing > programming. Side note: did we bring forward this feature of QVector to the Standardization Committee when alignas was introduced? >> In C++17 std::vector (well, std::allocator) must honour overly-aligned types: >> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0035r4.html > You might be right about that (hopefully), but the quoted document > mentions operator new with a new alignment capability, but container > allocators are not mentionned (AFAICS) Because it's a Standard and one needs to read it carefully; the change comes from the fact that std::allocator<T>::allocate (the allocation function of std::vector's default allocator) uses operator new (see [allocator.members]), and with the changes in that paper now both operator new and operator new[] must return aligned storage for overly-aligned types. HTH, -- Giuseppe D'Angelo | giuseppe.dang...@kdab.com | Senior Software Engineer KDAB (UK) Ltd., a KDAB Group company | Tel: UK +44-1625-809908 KDAB - Qt, C++ and OpenGL Experts
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