[Bug c++/23628] Typeinfo comparison code easily breaks shared libs

[mmarcus at emarcus dot org]

------- Additional Comments From mmarcus at emarcus dot org  2005-08-29 21:57 -------
Maybe I should rephrase my concern as the following question. Suppose a gcc 
user is building an application and some shared libraries that depend on third 
party libraries. Under what circumstances can he safely set visibility to 
someting other than default, for any types? My current understanding is that 
it is only safe to do so for types whose typeinfo will he can guarantee will 
never be shared. This is not always an easy guarantee to make, and so, if my 
undersatanding is correct, makes hidden visibility much less useful than it 
first appears. 

If typeinfo equality was based on string comparison instead of address 
comparison, the usefuleness of hidden visibility would be greatly increased. 
This would also address another important use case. There is often a desire to 
produce a shared library with only an extern "C" ABI, especially when building 
artifacts designed for long-term binary compatibility. That is, the only 
visible symbols should be extern "C" functions. To write such a shared library 
in C++, while depending upon a shared libstdc++ or other shared C++ libraries 
is not possible if one most make typeinfo visible just to share it with the 
libraries upon which our library depends. 

Address-only typeinfo equality comparison has a high usability cost. Would it 
be possible to first check for address equality, falling back to a string 
comparison upon failure?

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23628