TITLE: unary plus operator

(Newsgroups: comp.lang.c++,comp.lang.c++.moderated, 17 Nov 98)


WAHLE: "Peter J. Wahle" <p-wahle@uiuc.edu>

> In some code I'm working on I've found a typing error that is not flagged as
> an error when compiled (VC++ 5.0), and it seems to run correctly.  The
> typing error is the + sign on the first line of code below.  What exactly is
> this + sign doing?  Was it just ignored by the compiler or is it valid C++?
>
>  if( fabs(dC) <= accuracy ) {+
>    accuracy = fabs(dC);
>    return ZC;
>  }


CLAMAGE: Steve Clamage wrote in message <72ftq1$k9a$1@engnews2.Eng.Sun.COM>...

> >Unary plus is allowed as an operator, so the code is treated as
> >
> >    if( fabs(dC) <= accuracy ) {
> > + accuracy = fabs(dC);
> > return ZC;
> >    }
> >
> >But the unary plus turns the left side of the assignment into an
> >rvalue, so the assignment is technically disallowed. Evidently your
> >compiler is incorrectly treating the unary plus as having no effect.
> >(It has no effect, except for turning an lvalue into an rvalue
> >because the combination is an expression, not an object.)


SCHREIB: Dirk Schreib
> 
> Is an operator+() NOT allowed to return a non-const reference?


BONNARD: Valentin Bonnard <bonnardv@pratique.fr>

It is allowed to return anything (that can be returned), including 
for example a pointer to a function:

Foo f;
(+f) ();

Now, if accuracy is of type float, one obviously can't overload 
operator+ on it.

BTW, even if you return a rvalue of user defined type, 
that doesn't prevents it to be used on the left side of 
the assignement.