🚀 go-pugleaf

sakkinen@jyu.fi (Markku Sakkinen) writes:

:Inheritance between exceptions may be treacherous, since 'throw'
:does not work "virtually".  E.g., with Fergus's example:

:MyIntException mye (77);
:Exception *ep = mye;
:throw *ep;

:Only Exception is thrown!

Of course, that's no different from an ordinary call.  If you want
virtualness, you have to use pointers or references, as usual.

class Failure {
public:
  virtual int retry() = 0;
};

template < class T >
class MemFailure {
  int (*a_retry)(const T&);
  T a_context;
protected:
  virtual void get_more_memory () { free (some_extra_storage); }
public:
  MemFailure (int (*the_retry)(const T&), T the_context)
    : a_retry (the_retry), a_context (the_context) {}
  int retry() { get_more_memory (); return a_retry(a_context); }
};

void foo()
{
  try
    {
      bar ();
    }
  catch (Failure f)	// Oops!
    {
      if (f.retry ())	// Pure method call :-(
	throw;
    }
  try
    {
      bar ();
    }
  catch (Failure& f)	// OK!
    {
      if (f.retry ())
	throw;
    }
}
int my_retry (int)
{
  return 1;
}

void bar ()
{
  static MemFailure<int> mf(my_retry, 0);
  Failure& rf = mf;
  Failure* pf = &mf;
  switch (rand () & 1)
    {
    case 0:
      throw rf;	// OK!
      break;
    case 1:
      throw *pf;	// Oops, slicing :-(
      break;
    }
}

Niklas
-- 
Niklas Hallqvist	Phone: +46-(0)31-40 75 00
Applitron Datasystem	Fax:   +46-(0)31-83 39 50
Molndalsvagen 95	Email: niklas@appli.se
S-412 63  GOTEBORG, Sweden     mcsun!seunet!appli!niklas