Is it possible to use RTTI in a dynamic library ?

Is it possible to use RTTI in a dynamic library ?

[JOLY Loic]

The problem is under a Linux platform with gcc-3.2.2.
We've got an header containing the following (simplified):

class A
{
public:
    virtual bool fred1();
};

class B : public A
{
public:
    virtual bool fred2();
};

Included in a .so file (a dll in Windows terms)
and Included in an application

The .so file is loaded in a dynamic way: dlopen, dlsym, dlclose 
(LoadLibrary, GetProcAddress) and returns pointers to A objects.

What we want to do in the main application:

A *a = getFromSoFile(); // I'm sure this is a pointer to B in the real life
B *b = dynamic_cast<B *>(a);
--> The result is b == NULL;


Of course typeid(a) != typeid(B *)
but the strings returned by typeid(a).name() and typeid(B *).name() are 
equal.

Is there any way to make to make it work ? If we put everything directly 
in the program (no .SO file) it works as intended.


-- 
Loïc

Re: Is it possible to use RTTI in a dynamic library ?

[Nathan Sidwell]

JOLY Loic wrote:
> The problem is under a Linux platform with gcc-3.2.2.
> We've got an header containing the following (simplified):

> The .so file is loaded in a dynamic way: dlopen, dlsym, dlclose 
> (LoadLibrary, GetProcAddress) and returns pointers to A objects.
> 

> Is there any way to make to make it work ? If we put everything directly 
> in the program (no .SO file) it works as intended.
http://gcc.gnu.org/faq.html#dso

nathan
-- 
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          The voices in my head said this was stupid too
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk

Re: Is it possible to use RTTI in a dynamic library ?

[Eljay Love-Jensen]

Hi Loïc,

The RTTI mechanism used by dynamic_cast relies upon the address of the type information structure.  It appears that the DSO has a different type information structure for the object than the one being used in the dynamic_cast.  Not good.

What are the compiler/linker flags being used to make the .so?

Does -fweak make a difference?

If you are using -Bsymbolic ... don't use that.

Consider doing a reinterpret_cast<B*>, since you are sure that it is a pointer to B in real life.

Consider changing getFromSoFile() to return a B* instead of an A*.

--Eljay