RE: [Boost-Users] Problem with boost::any.

From: Jim.Hyslop (jim.hyslop_at_[hidden])
Date: 2002-10-11 08:40:57

Luc Bergeron [mailto:bluc_at_[hidden]] wrote:
> Greets,
> 
> I'm trying to use the boost::any component but i'm having 
> problem with 
> it !!!  I'm using it because I need a way to store parameters of 
> different type. That real fine but I can't retrieve the value 
> from the 
> boost::any. This is what i've tried :
> 
>    std::string GetStringFromAny(const boost::any& val) const
>    {
>       std::string strVal;
>       try
>       {
>          strVal = boost::any_cast<std::string>(val);
>       }
>       catch(boost::bad_any_cast &err)
>       {
>          std::cerr << err.what() << std::endl;
>          return "";
>       }
>       return strVal;
>    }
> 
> But I always have a bad_any_cast exception. From the 
> documentation, at 
> leats what I understand of it,  "If passed a value or reference, it 
> returns a copy of the value content if successful". Can 
> someone point me 
> in the right direction as to how can I extract the value within a 
> boost::any type, I think i'm lost !!! :)

The following program works for me, without any error (using MSVC6):

#include <boost/any.hpp>
#include <string>
#include <iostream>

std::string GetStringFromAny(const boost::any& val)
{
   std::string strVal;
   try
   {
      strVal = boost::any_cast<std::string>(val);
   }
   catch(boost::bad_any_cast &err)
   {
      std::cerr << err.what() << std::endl;
      return "";
   }
   return strVal;
}

int main()
{
  boost::any anAny( std::string("Hello, world") );
  std::string converted = GetStringFromAny( anAny );
  std::cout << "Converted: [" << converted << "]" << std::endl;
}

The output, as I expected, was:
Converted: [Hello, world]

Note that if I add more to main:

int main()
{
  boost::any anAny( std::string("Hello, world") );
  std::string converted = GetStringFromAny( anAny );
  std::cout << "Converted: [" << converted << "]" << std::endl;

  anAny = 23;
  converted = GetStringFromAny( anAny );
  std::cout << "Converted: [" << converted << "]" << std::endl;
}

the second conversion fails. The conversion will only succeed if the type
that boost::any currently holds can be implicitly converted into the type
you want to retrieve. If you want to convert from (for example) an int to a
string, you'll need boost::lexical_cast.

-- 
Jim
-- 
Jim