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From: tom@tnosoes.UUCP (Tom Vijlbrief)
Newsgroups: comp.lang.c++
Subject: Re^2: Is this a compiler bug ?
Keywords: G++
Message-ID: <438@tnosoes.UUCP>
Date: 30 Jan 89 16:05:08 GMT
References: <435@tnosoes.UUCP> <10022@diamond.csl.sony.JUNET>
Organization: TNO Institute for Perception, Soesterberg, The Netherlands
Lines: 51

diamond@csl.sony.JUNET (Norman Diamond) writes:

>In article <435@tnosoes.UUCP>, tom@tnosoes.UUCP (Tom Vijlbrief) writes:
>> When compiling a file of the OOPS package, G++ (1.32) produced an
>> errormessage for the following function:
>>         ((Date)*this).printOn(strm);    // Error !

>When you cast the result of *this, it is like assigning the
>result of *this to a temporary variable of type Date.  When
>you give a command to a temporary variable and then have no
>further use for that variable, you don't accomplish much.

This is not true. What is accomplished is that function Date::printOn
is executed for a temporay Date which is produced by converting
a Time object to a Date object. THIS IS WHAT IS INTENDED.

Instances of Time  (*this) and Date have different virtual functions printOn.

>>         ((Date *)this)->printOn(strm);  // No Error !

>Aha.  When you have sort of a temporary variable of type
>(Date *), it still points to your own instance.  You then
>command your own instance to do something, and you get
>what you want.

This statement does not produce the errormessage, but the result is
not what was intended. The type conversion from Time to Date is not
done.

This is a barebone copy of the error producing code:
==============================


class time;
 
class date {
        date(date&);
public:
        virtual void dummy() {};
};

class time {
        operator date();
        void f();
};


void time::f()
{
  ((date)*this).dummy(); // conv.c:17: both constructor and type conversion operator apply
}