[Mingw-notify] [ mingw-Bugs-568054 ] GCC 3.1 dynamic_cast fails with DLLs

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Bugs item #568054, was opened at 2002-06-13 00:02
You can respond by visiting: 
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Category: gcc
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Colin Peters (colinpeters)
Assigned to: Danny Smith (dannysmith)
Summary: GCC 3.1 dynamic_cast fails with DLLs

Initial Comment:
When attempting to use dynamic_cast in an 
executable to cast a base class pointer to an instance 
produced in a DLL to a derived class dynamic_cast 
fails, returning zero. e.g.

 class cFoo { ... };
 class cBar : public cFoo { ... };

 cFoo* pfoo = CreateBarInDLLAndCastToFoo();
 cBar* pbar = dynamic_cast<cBar*>(pfoo);

pbar ends up zero.

The same code works as expected when compiled into 
a single executable or when I revert to gcc 2.95.3 and 
an older version of binutils.

Attached is a simple test case demonstrating the 
problem. main.cpp is the executable, while foobar.cpp 
is built into a DLL. Doing a "make test" builds a static 
version static.exe and dynamic version dll.exe, then 
runs both. Sample output is included for make test 
with gcc 2.95.3 (old.out) and gcc 3.1 (new.out).

If the test works properly, the printed output for 
the "Bar pointer" will be non-zero in both static and 
DLL cases.

Note: The Makefile as it is builds with the -v flag. I also 
only used it under bash and with working versions of 
rm and echo available.


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