Xref: utzoo comp.lang.c++:12157 comp.std.c++:722
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From: jbuck@galileo.berkeley.edu (Joe Buck)
Newsgroups: comp.lang.c++,comp.std.c++
Subject: Re: distinguishing operator[] on left and right
Message-ID: <11874@pasteur.Berkeley.EDU>
Date: 11 Mar 91 18:54:00 GMT
References: <1991Mar6.235058.3641@osceola.cs.ucf.edu>
Sender: news@pasteur.Berkeley.EDU
Reply-To: jbuck@galileo.berkeley.edu (Joe Buck)
Lines: 63

In article <1991Mar6.235058.3641@osceola.cs.ucf.edu>, ssd@engr.ucf.edu (Steven S. Dick) writes:
|> What if I have a [] operator that does something unusual to extract the
|> data from the object... for instance, a bitfield...
|> 
|> // interface parts only...
|> class packedbits 
|> {
|>  public:
|>   packedbits(int size);
|>   int operator[](int index);
|>   void set(int index);
|>   void clear(int index);
|> };

Your problem is right there.  You're having operator[] return an int.  This
means that it can only be used as an lvalue and cannot be used to set the
bit.

Let's send a helper class to the rescue: change operator[](int) to return
a BitRef helper class:

class BitRef {
private:
	packedbits& pb;
	int index;
public:
	BitRef (packedbits& obj, int idx) : pb(obj), index(idx) {}
	operator int() { return pb.readBit(index);}
	BitRef& operator=(int newBit) {
		if (newBit) pb.set(index);
		else pb.clear(index);
		return *this;
	}
};

I need a new function in class packedbits: readBit(int) returns
the value of the bit at the given position.

Now when I say

	packedbits bitarray;

	int x = bitarray[23];

this turns into x = bitarray.readBit(23);

and

	bitarray[34] = x;

turns into

	if (x) bitarray.set(34); else bitarray.clear(34);

Note that the returned object acts like a reference to the
given bit.  In the case where the thing returned is an object,
we'd like to be able to redefine operator dot (to have a "smart
reference" class).  We can't with the ARM, though smart references
have been proposed as an extension to the language.

--
Joe Buck
jbuck@galileo.berkeley.edu	 {uunet,ucbvax}!galileo.berkeley.edu!jbuck