[Boost-users] can't extend boost::any on windows

Subject: [Boost-users] can't extend boost::any on windows
From: Michael Linck (mgl_at_[hidden])
Date: 2008-12-16 13:38:54

Hi, I've implemented a simple, low level json formatting utility to 
complement tinyjson's lightweight parser.  To do this, I've extended 
boost::any only to add a "format()" function to it that is aware of what 
types the content can take in a json structure and produces the 
appropriate string.  The default constructor and the constructor with a 
value are the only things I implemented, aside from the format 
function.  Both just call the base constructor.  Compiled in gcc, 
everything works fabulously.  Apparently, a functioning assignment 
operator and copy constructor are appropriately inherited and everything 
runs great.  On windows, however, some subtle difference causes 
assignment and copies to seg-fault.  The real problem is that, for the 
life of me, I can't find a way to implement assignment or copies 
explicitly, that doesn't also bomb out.  I can probably come up with a 
work around that stores my type in shared pointers so they're never 
actually copied, but I'm really curious if anyone could explain what 
causes this.  Here are some code snippets to illustrate the behavior:

void test(){
    std::cout<<"messing around with boost 
any"<<std::endl;                     
    boost::any 
tAny;                                                           
    boost::any tStringAny(std::string("some 
string"));                         
    tAny = 
tStringAny;                                                         
    std::cout<<"string any assignable to 
any"<<std::endl;                      
                                                                                

    std::cout<<"messing around with value 
type"<<std::endl;                    
    sysshep::json::Value 
tValue;                                               
    std::cout<<"formatted blank value 
"<<tValue.format()<<std::endl;           
    std::cout<<"making string 
value"<<std::endl;                               
                                                                                

    sysshep::json::Value 
tStringValue(std::string("some_string"));             
    std::cout<<"formatted string value 
"<<tStringValue.format()<<std::endl;    
    std::cout<<"assigning string value to blank 
value"<<std::endl;             
    tValue = 
tStringValue;                                                      /// FAILS
    std::cout<<"formatted blank value after 
assignment"<<tValue.format()<<std::endl;
}

class Value : public boost::any
{
    public:
       Value():boost::any(){};

       template<typename ValueTypeTA>
          Value(const ValueTypeTA & aValue):
          :          boost::any(aValue)       {}
   
        virtual ~Value();

        std::string format() const;
};

I'm not going to list the different ways I've attempted to implement 
copy and assignment, some more plausible than others, out of respect for 
brevity.  If you try a build on windows, VC8, you should be able to mess 
around with this to your hearts content.  It's really annoying because 
it was a factor in the fundamental elegance of the parsing 
implementation, because I see no reason why it shouldn't work, because 
it worked fine in linux, and because it put my windows port at least a 
day behind customer expectations.

thanks for any insight.