Thanks for the answer. > The compiler cannot warn, it has no way to know when compiling t1.cpp > that there's a different definition in t2.cpp, and vice versa. And what about lto (now it does not warn)? Kind regards, Markin Alex 2014-10-28 13:11 GMT+03:00 Jonathan Wakely <jwakely.gcc@xxxxxxxxx>: > [Redirected from gcc@] > > On 28 October 2014 09:33, Alex Markin wrote: >> As far as I understand the main issue here is that class A has >> external linkage and linker do not analyse the fields of class, it >> just watches class A by name. And the example itself breaks `3.2 One >> definition rule': > > That's correct. > >>> 5. >>> ... >>> — each definition of D shall consist of the same sequence of tokens >> >> >> And now my questions: >> >> * am I right with the understanding of situation and the example is UB? > > Yes > >> * if two definitions had the same fields and the same constructors, >> should the program be correct? > > It's not quite that simple. Every definition of the class must consist > of the same sequence of tokens, and all the names and functions it > refers to must resolve to the same entities in every translation unit. > The full rules are described in 3.2 [basic.def.odr] and are too long > to quote here. > > >> * is correct gcc behaviour with `-O1' a coincidence, or it detects >> another class anyway? > > The constructors get inlined into foo(void*) and bar(void*) > >> * does it make sense to give a warning in that case? > > The compiler cannot warn, it has no way to know when compiling t1.cpp > that there's a different definition in t2.cpp, and vice versa. > > The linker can warn, and the Gold linker has a --detect-odr-violations > option that warns about some cases. The linker is not part of GCC > though.
