A Convenient Exception Class

Hi,

When I create a class, I normally give it an exception Class -

class MyClass
{
  private:
    MyClassExpection(): public runtime_error
   {
    public:
      MyClassExpection(const string& what) :
        runtime_error("Exception within MyClass: " + what)
    {
    }
  };
};

I can then use this class as follows within MyClass scope -

stringstream failStringStream;
failStringStream << "The Step Id: " << (unsigned int)step << " is
invalid";
throw MyClassExpection(failStringStream.str());

But I was wondering if I could add another constructor to my exception
class as follows -

MyClassExpection(const stringstream& what) :
        runtime_error("Exception within MyClass: " + stringstream.str
())
    {
    }

and then use it like this -

throw MyClassExpection( stringstream() << "The Step Id: " <<
(unsigned int)step << " is invalid" );

or the less clear -

throw MyClassExpection( stringstream("The Step Id: ") << (unsigned
int)step << " is invalid" );

but these give a compiler error -

no matching function for call to
=91FS::MyClass::MyClassExpection::MyClassExpection
(std::basic_ostream<char, std::char_traits<char> >&)'

This does work -

stringstream failStringStream;
failStringStream << "The Step Id: " << (unsigned int)step << " is
invalid";
throw MyClassExpection(failStringStream);

why do I get this error?

are there any better solutions to the issue I'm trying to solve. That
is, the creation of a convenient exception object on one line.

Thanks,

Barry.