Exception class with shift operator

Hello,

I would like to use a shift operator for storing information
associated to an exception, as follows:

#include <stdexcept>
#include <string>

void foo()
{
  double x = 0.0;
  unsigned long u = 1;
  std::string s("Runtime error detected in ");

  // What I would like to do:
  throw std::runtime_error() << s << __FUNCTION__ << ":"
                             << " The value of x is " << x
                             << " and the value of u is " << u;
}

My trivial solution would be creating and using a class like the
following,
instead of std::runtime_error:

#include <exception>
#include <string>
#include <sstream>

namespace ext
{
  class runtime_error : public std::exception
  {
  public:
    runtime_error() : my_ss() {}
    explicit runtime_error(const std::string& arg) : my_ss(arg) {}
    runtime_error(const runtime_error& e) : my_ss(e.what()) {}
    runtime_error& operator=(const runtime_error& e)
    {
      std::string s(e.what());
      my_ss.clear();
      my_ss << s;
      return *this;
    }

    virtual ~runtime_error() throw() {}

  public:
    virtual const char* what() const throw()
    { return my_ss.str().c_str(); }

    template <typename T> runtime_error& operator<<(const T& v)
    { my_ss << v; return *this; }

  private:
    std::stringstream my_ss;
  };

} // namespace ext

Can you see problems related to this class and its usage for the
purpose mentioned above?
How can I make it better or, are there other possible solutions?

Thanks in advance for your opinions,
eca

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