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operator==() of the type_info class
- To: gcc at gcc dot gnu dot org
- Subject: operator==() of the type_info class
- From: Jörg Budischewski <Joerg dot Budischewski at sun dot com>
- Date: Thu, 04 Oct 2001 14:12:39 GMT
my name is Joerg Budischewski, I am a Sun Microsystems employee working
on OpenOffice (see http://www.openoffice.org) / StarOffice. I work on
UNO, the openoffice component model.
There is currently some effort underway to port the linux build of
openoffice from gcc 2.95.2 to gcc 3.01. The results of the build are
currently very promising (some shared libraries have shrinked in size by
50-60%, you have done a great job !), though I can't give a final
statement as the build has not finished yet (we e.g. need to change lots
of code lines due to the harder check for exception specifications).
However, we have run into a problem I would like to discuss here.
The throwing of exceptions via shared library boundaries only works, when
the shared library exports the exception's typeinfo symbol. This is due
to that the type_info::operator==() is implemented by comparing the
type_info-instances-pointers instead of comparing the type-names.
In the version 2.95.2, there was an additional strcmp() for the names
after the pointer comparision.
In our case, we want to limit the exports of shared libraries to the
absolute minimum (which are 3 C-functions for each shared library in our
case). So every library has its own instance of the exception-typeinfo.
This leads to the unexpected exception.
Though we could solve this by additinal exporting every type_info
exception symbol, there may still arise a problem when the first library
offering a certain exception symbol gets unloaded (dlclose()). The
type_info for this certain type is then not available for other libraries
anymore which may lead to segfaults.
So I would like to raise the question, why the strcmp() was removed ? The
only thing I can think of is performance and codesize of the inline
function, but I think both have only neglectable effect in case of
Greetings, Joerg Budischewski
PS: If would like to cite Bjarne Stroustrup (The C++ programming
language, 3rd edition, chapter 15.4.4 Typeid and extended type
information ) on this topic :
'It is not guaranteed that there is only one type_info object for each
type in the systen. In fact, where dynamically linked libraries are used,
it can be hard for an implementation to avoid duplicate type_info
objects. Consequently we should use == on type_info objects to test
equality, rather than == on pointers to such objects.'