bad_any_cast being thrown but no idea why

[John Yamokoski]

I have been pulling my hair out over this bug for a day or two now.
Here's the synopsis. I have a boost::any object which is storing a
std::string. Any test I do to check if indeed the any object is
holding onto a std::string object verifies what I already know. For
instance, the following function always returns true for my boost::any
object:

bool is_string(const boost::any& operand)
{
        return ( operand.type() == typeid(std::string) );
}

Also if I just print out the typeinfo information for the boost::any
object is also shows that its holding onto a std::string:

std::stringstream ss;
ss << "any object: " << obj.type().name() << ", a string: " <<
typeid(std::string).name();

yields,

any object: Ss, a string: Ss

However, whenever I try and actually cast my object to a string
(boost::any_cast<std::string>(...)), I always get
"boost::bad_any_cast: failed conversion using boost::any_cast".

Could this possibly be a compiler issue? I saw one other thread where
"-fvisibility=hidden" was causing some problems for
boost::program_options and std::string. In my situation, I have a
library which is responsible for creating those boost::any objects.
Then its in a separately compiled library where I am getting the
problems.
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[Michael Caisse-2]

John Yamokoski wrote:

> I have been pulling my hair out over this bug for a day or two now.
> Here's the synopsis. I have a boost::any object which is storing a
> std::string. Any test I do to check if indeed the any object is
> holding onto a std::string object verifies what I already know. For
> instance, the following function always returns true for my boost::any
> object:
>
> bool is_string(const boost::any& operand)
> {
> return ( operand.type() == typeid(std::string) );
> }
>
> Also if I just print out the typeinfo information for the boost::any
> object is also shows that its holding onto a std::string:
>
> std::stringstream ss;
> ss << "any object: " << obj.type().name() << ", a string: " <<
> typeid(std::string).name();
>
> yields,
>
> any object: Ss, a string: Ss
>
> However, whenever I try and actually cast my object to a string
> (boost::any_cast<std::string>(...)), I always get
> "boost::bad_any_cast: failed conversion using boost::any_cast".
>
> Could this possibly be a compiler issue? I saw one other thread where
> "-fvisibility=hidden" was causing some problems for
> boost::program_options and std::string. In my situation, I have a
> library which is responsible for creating those boost::any objects.
> Then its in a separately compiled library where I am getting the
> problems.
> _______________________________________________
>  

Hi John -

The problem is that the RTTI id's are different between your lib
and the executable. You can use the undocumented and very useful
unsafe cast:

        boost::unsafe_any_cast< std::string>( ... )

Of course, this creates a problem since it is not safe and if the
any doesn't contain a std::string there will be a surprise.

I think the typical way of handling this (or at least what I do)
is something like this:

----------------------

template< typename T > bool contains( const boost::any& a )
{
   try
   {
      // we do the comparison with 'name' because across shared library boundries we get
      // two different type_info objects
      return( std::strcmp( typeid( T ).name(), a.type().name() ) == 0 );
   }
   catch( ... )
   { }

   return false;
}

...

if( contains< std::string >( foo ) )
{
   bar = boost::unsafe_any_cast< std::string >( &foo );
}

-------------------------


Hope this helps you out.
michael

--

----------------------------------
Michael Caisse
Object Modeling Designs
www.objectmodelingdesigns.com


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[John Yamokoski]

>>
>> I have been pulling my hair out over this bug for a day or two now.
>> Here's the synopsis. I have a boost::any object which is storing a
>> std::string. Any test I do to check if indeed the any object is
>> holding onto a std::string object verifies what I already know. For
>> instance, the following function always returns true for my boost::any
>> object:
>>
>> bool is_string(const boost::any& operand)
>> {
>>        return ( operand.type() == typeid(std::string) );
>> }
>>
>> Also if I just print out the typeinfo information for the boost::any
>> object is also shows that its holding onto a std::string:
>>
>> std::stringstream ss;
>> ss << "any object: " << obj.type().name() << ", a string: " <<
>> typeid(std::string).name();
>>
>> yields,
>>
>> any object: Ss, a string: Ss
>>
>> However, whenever I try and actually cast my object to a string
>> (boost::any_cast<std::string>(...)), I always get
>> "boost::bad_any_cast: failed conversion using boost::any_cast".
>>
>> Could this possibly be a compiler issue? I saw one other thread where
>> "-fvisibility=hidden" was causing some problems for
>> boost::program_options and std::string. In my situation, I have a
>> library which is responsible for creating those boost::any objects.
>> Then its in a separately compiled library where I am getting the
>> problems.
>> _______________________________________________
>>
>
> Hi John -
>
> The problem is that the RTTI id's are different between your lib
> and the executable. You can use the undocumented and very useful
> unsafe cast:
>
>        boost::unsafe_any_cast< std::string>( ... )
>
> Of course, this creates a problem since it is not safe and if the
> any doesn't contain a std::string there will be a surprise.
>

Michael,

Well its great to hear there's some sort of explanation for this.
Unfortunately, from my compiler, I get:

no matching function for call to 'unsafe_any_cast(const boost::any&)'

I believe I have installed Boost version 1.40. I will gladly downgrade
to an earlier version if unsafe_any_cast was finally eliminated in
1.40. Otherwise is there another way to deal with RTTI ID issues? This
is the first I have heard of them. Ahhh.. so close though.

J.D.
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[Michael Caisse-2]

John Yamokoski wrote:

>> Hi John -
>>
>> The problem is that the RTTI id's are different between your lib
>> and the executable. You can use the undocumented and very useful
>> unsafe cast:
>>
>>        boost::unsafe_any_cast< std::string>( ... )
>>
>> Of course, this creates a problem since it is not safe and if the
>> any doesn't contain a std::string there will be a surprise.
>>
>>    
>
> Michael,
>
> Well its great to hear there's some sort of explanation for this.
> Unfortunately, from my compiler, I get:
>
> no matching function for call to 'unsafe_any_cast(const boost::any&)'
>
> I believe I have installed Boost version 1.40. I will gladly downgrade
> to an earlier version if unsafe_any_cast was finally eliminated in
> 1.40. Otherwise is there another way to deal with RTTI ID issues? This
> is the first I have heard of them. Ahhh.. so close though.
>  

John -

It is still in the interface:

https://svn.boost.org/trac/boost/browser/tags/release/Boost_1_40_0/boost/any.hpp#L219

Did you remember the template parameter?

        boost::unsafe_any_cast< std::string>( ... )

Do you have a small code snippet that reproduces the problem?

michael


--

----------------------------------
Michael Caisse
Object Modeling Designs
www.objectmodelingdesigns.com


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[John Yamokoski]

>>>
>>> Hi John -
>>>
>>> The problem is that the RTTI id's are different between your lib
>>> and the executable. You can use the undocumented and very useful
>>> unsafe cast:
>>>
>>>       boost::unsafe_any_cast< std::string>( ... )
>>>
>>> Of course, this creates a problem since it is not safe and if the
>>> any doesn't contain a std::string there will be a surprise.
>>>
>>>
>>
>> Michael,
>>
>> Well its great to hear there's some sort of explanation for this.
>> Unfortunately, from my compiler, I get:
>>
>> no matching function for call to 'unsafe_any_cast(const boost::any&)'
>>
>> I believe I have installed Boost version 1.40. I will gladly downgrade
>> to an earlier version if unsafe_any_cast was finally eliminated in
>> 1.40. Otherwise is there another way to deal with RTTI ID issues? This
>> is the first I have heard of them. Ahhh.. so close though.
>>
>
> John -
>
> It is still in the interface:
>
> https://svn.boost.org/trac/boost/browser/tags/release/Boost_1_40_0/boost/any.hpp#L219
>
> Did you remember the template parameter?
>
>        boost::unsafe_any_cast< std::string>( ... )
>
> Do you have a small code snippet that reproduces the problem?
>
> michael
>

Got it! I wasn't passing in a pointer to the any object.. it should have been:

boost::unsafe_any_cast< std::string>( &obj )

Now its working! Michael thanks so much - you have saved some of my hair! ;)

btw, still curious about this RTTI stuff. Is there another way to
handle this other than doing unsafe casts?

J.D.
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