Can I check the type of a void pointer

[Damon]

Hi,

I created a template class and stored its instances in a map object
that is like std::map<std::string, void* >. But i just made a nasty
discovery that if i fail to use a C-style cast properly with the void
pointer to return it to the proper class it belongs to, I get very bad
and random segmentation fault.

So question to all the experts, can I check the type of a void pointer
or use some guaranteed way to elicit an exception without going
segmentation fault? BTW, I'm using g++ v3.2.2. I know that if i try to
delete the instances of the template class that was cast wrongly, it
segfault but I can't trap it!

Thank you in advance! Merry X'mas to everyone.

Regards,
Damon

 

[Ron Natalie]

So question to all the experts, can I check the type of a void pointer
or use some guaranteed way to elicit an exception without going
segmentation fault? BTW, I'm using g++ v3.2.2. I know that if i try to
delete the instances of the template class that was cast wrongly, it
segfault but I can't trap it!

No, once you cast a pointer to void*, you're only options are:
check it against the null pointer constant.
cast it back to exactly what it was before.

The only solution I can suggest is to make your template class inherit
from a common (public) base class and use that rather than void*. If
the base class is polymorphic (has a virutal function), then you can use
dynamic_cast to validate it against any particular derived class.

 

[Martijn Lievaart]

No, once you cast a pointer to void*, you're only options are:
check it against the null pointer constant.
cast it back to exactly what it was before.

The only solution I can suggest is to make your template class inherit
from a common (public) base class and use that rather than void*. If
the base class is polymorphic (has a virutal function), then you can use
dynamic_cast to validate it against any particular derived class.

Adding to that, if you /need/ a void pointer (say for some C-ish
callback), you can use this technique as well. Cast to the base class.
Cast to void. Pass pointer. Cast back to base class. Use polymorphism
(virtual functions) or dynamic_cast<> from there.

HTH,
M4

 

[tom_usenet]

Hi,

I created a template class and stored its instances in a map object
that is like std::map<std::string, void* >. But i just made a nasty
discovery that if i fail to use a C-style cast properly with the void
pointer to return it to the proper class it belongs to, I get very bad
and random segmentation fault.

So question to all the experts, can I check the type of a void pointer
or use some guaranteed way to elicit an exception without going
segmentation fault? BTW, I'm using g++ v3.2.2. I know that if i try to
delete the instances of the template class that was cast wrongly, it
segfault but I can't trap it!

A void* doesn't include any info about the type of the object pointed
to. A boost::any does: std::map<std::string, boost::any>. See
www.boost.org. Alternatively, redesign and use something better than a
void* (such as a base class with virtual functions). There's rarely
any good reason to use something like boost::any.

Tom

C++ FAQ: http://www.parashift.com/c++-faq-lite/
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html

 

[Jumbo]

Hi,

I created a template class and stored its instances in a map object
that is like std::map<std::string, void* >. But i just made a nasty
discovery that if i fail to use a C-style cast properly with the void
pointer to return it to the proper class it belongs to, I get very bad
and random segmentation fault.

So question to all the experts, can I check the type of a void pointer
or use some guaranteed way to elicit an exception without going
segmentation fault? BTW, I'm using g++ v3.2.2. I know that if i try to
delete the instances of the template class that was cast wrongly, it
segfault but I can't trap it!

Thank you in advance! Merry X'mas to everyone.

Your a bit early aren't you? ) Merry Christmas to you too.

Have you tried STL <typeinfo>
catch(bad_typeid)

HTH

 

[Greg Comeau]

Your a bit early aren't you? ) Merry Christmas to you too.

Have you tried STL <typeinfo>
catch(bad_typeid)

He might be able to "record" typeid's but as per the other responses
there are more "programmatic" ways to approach the problem, and
they will naturally adapt automatically for him too.

 

[Damon]

Thanks to all. It's an eye opener.