Re: [Boost-users] bad_any_cast being thrown but no idea why

Subject: Re: [Boost-users] bad_any_cast being thrown but no idea why
From: John Yamokoski (yamokosk_at_[hidden])
Date: 2009-10-31 21:58:03

>>
>> 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.