RE: [boost] Re: Standard C Library and C++ / BOOST

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-03-27 11:11:21

Bo Persson wrote:
>"Reece Dunn" <msclrhd_at_[hidden]> skrev i meddelandet
>news:BAY7-F123eo6WutfjDM00010dd8_at_hotmail.com...
> > Klaus Nowikow wrote:
> > >Reece Dunn wrote:
> > >[snip]
> > > > A trick I use when handling HRESULT error codes is to have a
> > > > class like this
> > > > (adapted for int error type):
> > > >
> > > >    class errorcheck
> > > >    {
> > >[snip]
> > > >       public:
> > > >          inline errorcheck( int ec ): error( ec )
> > > >          {
> > > >             if( ec < 0 ) throw( *this );
> > > >          }
> > >
> > >Careful. Throwing an exception from a constructor
> > >means that the object won't be constructed (i. e., does not exist).
> > >So you are throwing a non-existing object here.
> > >
> > >Or am I wrong?
> >
> > If I've got the standard correct, the object will be partially 
>constructed
> > up to the point where the exception is thrown. Thus, if this throws an
> > exception, the error member will be set.
>
>No, when the constructor fails any partial construction will be undone by
>calling the destructors for those subobjects.

So the errorcheck object will be in a half-constructed state. That is, the 
int will be constructed, but the errorcheck object isn't, therefore the 
int's destructor will be called, but errorcheck's deconstructor will not.

The solution would therefore be to throw an intermediate object, like the 
Trules from the C++ Templates book by David Vandervoorde and Nico Josuttis. 
This would then get around the throwing in a constructor problem.

Regards,
Reece

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