On Fri, 24 Feb 2023 at 09:49, Jakub Jelinek wrote: > > Assuming a compiler handles the T m_vecdata[1]; as flexible array member > like (which we need because standard C++ doesn't have flexible array members > nor [0] arrays), I wonder if we instead of the m_auto followed by m_data > trick couldn't make auto_vec have > alignas(vec<vl_embed>) unsigned char buf m_data[sizeof (vec<vl_embed>) + (N - > 1) * sizeof (T)]; > and do a placement new of vec<vl_embed> into that m_data during auto_vec > construction. Isn't it then similar to how are flexible array members > normally used in C, where one uses malloc or alloca to allocate storage > for them and the storage can be larger than the structure itself and > flexible array member then can use storage after it?
You would still be accessing past the end of the vec<vl_embed>::m_vecdata array which is UB.
