Re: [boost] Plea for addition of stream_cast

From: Peter Dimov (pdimov_at_[hidden])
Date: 2000-06-06 10:52:00

> > > Returning default values on failure is ambiguous, unless you
> > > know from your context that the default would be impossible.
> >
> > Agreed.  But sometimes we may indeed know this:
> >
> >       int n = stream_cast<int>(argv[1], 0);
> >       if (n < 5)
> >       {
> >           std::cerr << "x must be a number greater than 4.";
> >           return EXIT_FAILURE;
> >       }
> >
> > Do we care whether the user typed "fred" or "3"?  Possibly not.
>
> This type of error handling scenario is a poster child for exceptions.
> The exception overhead is insignificant, and it makes easy generic and
> more robust error handling.  True, you don't need to tell to the user
> that the parameter wasn't even a number, but sometimes it can be nice.
> With exceptions, you can use the same code, or at least the same
> coding pattern, for problems that do and do not have the luxury of
> knowing that 0 (or whatever) is illegal and for problems that do and
> do not need verbose error reporting.

The point is that the programmer should make the decision whether a default
return value or an exception is more appropriate.

When faced with an exception-throwing stream_cast, I would be forced to
define my own

template<class T, class S> T stream_cast_def(S const & source, T const &
defaultValue = T())
{
    try
    {
        return stream_cast<T>(source);
    }
    catch(stream_cast_exception const &)
    {
        return defaultValue;
    }
}

which is, in this case, inefficient.

--
Peter Dimov
Multi Media Ltd.