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From: dl@g.g.oswego.edu (Doug Lea)
Newsgroups: comp.lang.c++
Subject: Re: operator []: ref vs value
Message-ID: <DL.90Feb25100053@g.g.oswego.edu>
Date: 25 Feb 90 15:01:04 GMT
References: <5216@brazos.Rice.edu> <25E5841D.5B4A@tct.uucp>
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Reply-To: dl@oswego.edu
Organization: SUNY Oswego
Lines: 43
In-reply-to: chip@tct.uucp's message of 23 Feb 90 18:42:36 GMT


> From: chip@tct.uucp (Chip Salzenberg):
> 
> According to dougm@zamenhof.rice.edu (Doug Moore):
> >Unfortunately, I want one behavior when the pseudo-array access occurs
> >as an rvalue and another when it appears as an lvalue.  Can I get what
> >I want?
> 
> You almost had it.  Define a reference_to_missing_array_element type,
> and have operator[] return an object of that type.  Then define:
> 
> 	reference_to_missing_array_element::operator float()
> 
> 		return 0.0
> 
> 	reference_to_missing_array_element::operator = (float)
> 
> 		create the missing array element and assign to it
> 
> No sweat.
> 

Consider what happens in

void inc(float& f) { f += 1.0; }

main()
{
   // ...
  inc(v[0]);
}

Here, v[0] returns a reference_to_missing_array_element, which is
`coerced' to return a constant value (0.0). The 0.0 is made into a
temporary variable in order to make a reference. This temporary is
then modified in inc(), without touching the underlying representation
in v. This is probably not what a user would have in mind. 

-Doug
--
Doug Lea, Computer Science Dept., SUNY Oswego, Oswego, NY, 13126 (315)341-2367
email: dl@oswego.edu              or dl%oswego.edu@nisc.nyser.net
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