Hello, I am trying to use dynamic_cast to upcast object pointers generated by object factories. Each factory belong to a separate dynamic shared object (OSX bundles in this case), and GCC fails the cast in few cases (and succeeds in the others). In order to understand the problem, I've created a factory that creates an empty object (only a CTOR and a virtual DTOR). When the destructor is inline, everything works fine (the CTOR's definition is in a .cpp). However, as soon as I move the destructor's definition to the .cpp, the dynamic_cast fails. I looked at the symbols in each case (nm -m <bundle-file>) and found that the only differences were that the symbols associated with the empty class are declared weak extern in the binary where the destructor is inline and extern in the other binary. Here is my understanding of the problem: dynamic_cast can only succeeds in GCC if both object's typeinfo have the same address. If there are several definitions of the same class in different DSOs, then some of the duplicated symbols associated to those definitions have to be marked as weak in order to be coalesced to enforce the one definition rule (ODR). This is exactly what doesn't happen when my empty class has its constructor defined in the .cpp. I've stumbled on a http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15428#c0 bug report relating to GCC on Darwin which adjusted GCC's behavior regarding vtables to compensate a restriction from the linker on Darwin. Because of this change, each duplicated symbol related to an empty object is marked as non-weak and will have its own typeinfo object (and its own typeinfo pointer), which will cause dynamic_cast to fail on GCC. In order to revert this behavior, I need to have a 'key function' in my class. So here are my two questions: What qualifies as a key function (I've tried several dummy function prototypes, but none solved my problem)? Where can I find some guidelines about the way I should code my classes so that gcc marks symbols as weak on Darwin? I've looked for answers for both questions for several days with no success. Your help will be greatly appreciated. Thank you very much in advance, Nicolas -- View this message in context: http://old.nabble.com/Question-about-Darwin-GCC-and-weak-symbol-tagging-policy-tp30654362p30654362.html Sent from the gcc - Help mailing list archive at Nabble.com.
