Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!mailrus!tut.cis.ohio-state.edu!bloom-beacon!gatech!gt-eedsp!baud
From: baud@gt-eedsp.UUCP (Kurt Baudendistel)
Newsgroups: comp.lang.c++
Subject: Re: matrix types and [] overloading
Summary: a supportive response
Keywords: matrix, [] overloading
Message-ID: <538@gt-eedsp.UUCP>
Date: 20 Oct 88 15:17:50 GMT
References: <5181@saturn.ucsc.edu>
Reply-To: baud@gt-eedsp.UUCP (Kurt Baudendistel)
Organization: School of Electrical Engineering, Ga. Tech, Atlanta, GA  30332
Lines: 35

In article <5181@saturn.ucsc.edu> skinner@saturn.ucsc.edu (Robert Skinner) writes:
>I'm constructing a general 2d Matrix object for a class I'm now taking
>on matrix computations.  It would be nice if I could
>overload the [] operator to allow something like a[1][2], but I
>understand that is not possible.  

you should understand that you CAN do this if you want:

1. a is of type MATRIX.
2. define MATRIX::operator[] to return a value of type VECTOR.
3. define VECTOR::operator[] to return a value of type float.

the concept is simple. the details are tricky.

>Another option is to overload the () operator to provide array indexing:
>
>	virtual float	*operator()( int i, int j ) 
>				{ return data + i*cols + j; }
>

i don't really understand where ``all the *'s in the code'' come from,
but what you really want to do is return a reference to the value:

float& operator()( int i, int j ) 
  { return *(data + i*cols + j); }

even better, watch for the prototypable matrix class to be included soon
in the libg++ distribution (it comes with g++, but could be adapted by
someone for general c++ use).

kurt
-- 
Kurt Baudendistel [GRA McClellan]
Georgia Tech, School of Electrical Engineering, Atlanta, GA  30332
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