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From: spencer@egypt.mpr.ca ( Reid Spencer)
Newsgroups: comp.lang.c++
Subject: Re: conversions
Message-ID: <2141@kiwi.mpr.ca>
Date: 13 Apr 90 00:57:50 GMT
References: <7633@cadillac.CAD.MCC.COM>
Sender: news@eric.mpr.ca
Reply-To: spencer@egypt.mpr.ca ( Reid Spencer)
Organization: Microtel Pacific Research Ltd. (MPR)
Lines: 63


In article <7633@cadillac.CAD.MCC.COM>, vaughan@puma.cad.mcc.com (Paul
Vaughan) writes:
> 
> The following program compiles cleanly under g++-1.37.1, but gets
> three errors under cfront (Sun CC, actually).
> 
example omitted.
> 
> Any comments on why CC generates these errors, and whether or not this
> is the right thing to do?  I realize that a class that works Exactly
> like an int for both reading and assignment probably isn't very
> useful, but should I be able to do it this way?
> 

I'm not sure what you were intending with this example, but at first
appearances it seems you
simply forgot to override the "+=" operator for class "Int". I think,
perhaps that you expected
cfront to use the constructor to add the value of i to itself. However,
even if that were semantically
possible it wouldn't provide the results you want (i.i being 8, not 4).
Why g++ accepts this program,
I don't know. It's probably a bug in g++. cfront seems to be giving you
the right messages.
Here's the program with some modifications I made:

------------------------------------------
extern "C" { int printf(char *, ...); };			// new

class Int {
  int i;
  public:
    Int(int j) : i(j) {}
    Int(Int& I) { i = I.i; }
    operator int() { return i; }
    operator int&() { return i; }
    Int & operator +=(Int &j) { i += j.i; return *this; }	// new
    Int & operator +=(int j) { i += j; return *this; }		// new
};

main() {
  Int i = 4;
  i += i;               // line 29
  i += 1;							// new
  printf("i.i is %d\n", int(i));				// new
}
----------------------------------------------

The printf statement should print "i.i is 9". This kind of problem is a
common misperception in C++.
Please make note of the following statement: "the semantics of
initialization and assignment are
different". See Lippman's book "C++ Primer", pp. 243 and 268 for
details. Also in Stroustrup, pp 178-180.

Hope this helps ...
_______________________________________________________________________________
Reid Spencer
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