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From: bytor@ctt.bellcore.com (Ross Huitt)
Newsgroups: comp.lang.c++
Subject: Re: Question about type conversion.
Summary: use of cast operator
Message-ID: <1991May2.163512.8587@bellcore.bellcore.com>
Date: 2 May 91 16:35:12 GMT
References: <9580001@hpdtczb.HP.COM>
Sender: usenet@bellcore.bellcore.com (Poster of News)
Organization: Computer Technology Transfer, Bellcore
Lines: 55

In article <9580001@hpdtczb.HP.COM> dent@hpdtczb.HP.COM (Steve Dent) writes:
//....

>   One of the old C 
>routines returns a variable which may have multiple types: i.e an integer,
>a double, or a pointer to a character.  The user of this routine is 
>provide conversion macros to convert the variable to a known type.  The 
>definition of the return structure is as follows:
>
>struct FIELD
>{
>   int type;
>   void *fieldValue;
>}

//....

>What I would like to do is create a C++ wrapper around this structure which
>will take care of conversion for the user.  So instead of:
>	var1 = convInt(ret1);
>It would look like this:
>	var1 = ret1;

>The real question is how do I inform the C++ compiler of type conversion rule
>from a user defined class to a built in type?  Is this possible?
>
>Steve Dent
>Design Technology Center
>Hewlett Packard

How about something like:

struct FIELD
{
   int   type;
   void *fieldValue;
   inline operator int(void)    { return *(int*)fieldValue; }
   inline operator double(void) { return *(double*)fieldValue; }
   inline operator char *(void) { return (char *)fieldValue; }
};

// this would let you do something like:

FIELD f;
int x = f;
double y = f;
char *s = f;


However, I try to avoid cast operators for my classes unless I have
a _very_ _very_ good reason for doing so.  They can be very deadly
because they may get invoked where you don't intend.  This example
looks harmless enough, though. :-)

-bytor