Geoffrey Keating <[EMAIL PROTECTED]> writes: [...]
| > | I don't think you can say 'no piece of code knows about both of | > them'. | > | What you can say is that these two classes are both named S but | > | they're different, just as if they were in different namespaces. | > | > That would mirror how C++ handles classes in unnamed namspaces. In | > other words, the visibility would have to be part of the mangled name. | | Well, yes and no. Actually, what I'd like C++ to do is to not have | the mangled name for an anonymous namespace object contain the random | string it currently contains. I think Jason's changes now let me do | this! I'm all for the any predictable scheme that makes the mangled unique. What I meant by "that would mirror the unnamed namespace case" is that that no longer violates the ODR rule. It makes the case defined. | > | > On Thu, Jul 13, 2006 at 03:41:29PM +0200, Gabriel Dos Reis wrote: | > | > > I'm not clear about "you can't compare them". | > | > > | > | > > Surely, I can take the address of typeid(S) and pass it | > around to | > | > > a function in another translation unit. I can do | > | > > typeinfo1->before(*typeinfo2), where typeinfo1 and typeinfo2 | > comes | > | > > from two such different translation units. | > | > > | > | > > How the current visibility framework prevent that from | > happening? | > | > | > | > By a note in the documentation telling the user "don't do that". | > | | > | No, there's no such note. The answer is that the two typeids have | > | different addresses, so one will be before the other, depending on | > | where the shared libraries got loaded, just as if the classes had | > | different names. | > | > In the case where __GXX_MERGED_TYPEINFO_NAMES is not defined, that | > would not be true. Because the current definition of | > type_info::before uses a strcmp() on their mangled name. | > To make the implementation correct, the mangled name need to | > incorporate the visibility attribute. | | I'm not sure what you mean by the 'visibility attribute'. I think | that to make this work, you would need to have the mangled name | contain a unique identifier (perhaps its load address) for the shared | object instance in question, just as for anonymous namespace names we | have the name contain a unique identifier for the translation unit in | question. | | However, getting this unique identifier is going to be a little | tricky. You can't do it at compile time (since this .o file might be | in more than one shared object) or at link time (since this shared | library might be loaded more than once, for instance using dlopen ()). Yes. | So it has to be set up at load time. So we're talking about strings | in writable memory, linker and assembler and compiler changes to | generate the relocations, binary incompatibility... It seems like it | would be just as easy to implement weak symbols and then have | __GXX_MERGED_TYPEINFO_NAMES to be defined. | | I don't believe there's any platform yet which has visibility but not | weak symbols, so this is not a problem that needs to be solved at the | moment. I seem to remember a PR posted by Adobe people kind of related to this, but maybe I'm remembering wrong. I have to dig up bugzilla. -- Gaby
