Path: utzoo!attcan!uunet!samsung!zaphod.mps.ohio-state.edu!van-bc!ubc-cs!uw-beaver!ssc-vax!fluke!dyndata!dan
From: dan@dyndata.UUCP (Dan Everhart)
Newsgroups: comp.lang.c++
Subject: implicit conv to int in subscript
Message-ID: <684@dyndata.UUCP>
Date: 25 May 90 05:38:17 GMT
Organization: Dynamic Data & Electronics, Edmonds WA
Lines: 49


Consider the following program:

class Index
   {
public:
   Index (int i) : v (i) {};
   operator int ()   { return v; };
private:
   int   v;
   };

Index x (0);
char  ac [10];

f ()
   {
   ac [x] = 'a';                // this line fails
   
   ac [(int) x] = 'a';
   ac [int (x)] = 'a';
   }


On the marked line, I expected the compiler to make an implicit
conversion of x to int, since the Index class has an operator int ()
function defined.  Zortech 2.06 gives the error message:

"opint.cpp", line 17 Syntax error: illegal pointer arithmetic
Had: <near *><char>
and: <struct Index>

The subsequent two lines, with their explicit conversions, compile
fine. 

Although I could not find any example in The C++ Primer of exactly
this nature, there is a close one on p. 283 where Lippman defines a
SmallInt class, with similar conversion operators and says, "A
SmallInt class object can now be used *anywhere* in int object can be
used."  (Emphasis mine.)  

Taking a clue from the error message, I also tried adding the line

	friend char & operator + (char *, Index);

to the class definition, attempting to provide assistance with the
pointer arithmetic.  It made no difference.

So, should this work?