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From: dent@hpdtczb.HP.COM (Steve Dent)
Newsgroups: comp.lang.c++
Subject: Question about type conversion.
Message-ID: <9580001@hpdtczb.HP.COM>
Date: 30 Apr 91 00:20:36 GMT
Organization: HP Design Tech Center - Santa Clara, CA
Lines: 52

Sorry If this is question has been asked before.

I'm developing a program which using existing C routines.  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;
}


A conversion macro would look like this:

#define convInt(a) (*(int *) a->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;

I tried to create an overloaded operator= function as follows:

inline
void operator=(int tmp, FIELD *val)
{
    tmp = (*(int *) val->fieldValue);
}

NOTE:  this is simplified for an example.

The problem is that the ATT 2.1 compiler gives me the following warning:

CC: "s1.C", line 87: warning: non-member operator =() (anachronism) (1531)

Which after looking up means that overloading the operator=() will only be 
supported as a member functions in future releases of the C++ language.  
I'm really trying to archive is type conversion not an assignment. 

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