Path: utzoo!news-server.csri.toronto.edu!rutgers!soleil!mlb.semi.harris.com!trantor.harris-atd.com!jedi!mvm
From: mvm@jedi.harris-atd.com (Matt Mahoney)
Newsgroups: comp.lang.c++
Subject: Re: Beginners problems (longish)
Message-ID: <5829@trantor.harris-atd.com>
Date: 14 Mar 91 21:02:14 GMT
References: <1991Mar13.170912.4020@cs.UAlberta.CA>
Sender: news@trantor.harris-atd.com
Reply-To: mvm@jedi.UUCP (Matt Mahoney)
Organization: CAE Design Center, Harris Corp., Melbourne, Fl.
Lines: 43

In article <1991Mar13.170912.4020@cs.UAlberta.CA> dwelly@saddle-lk.cs.UAlberta.CA (Andrew Dwelly) writes:
>
>Would someone take pity on me and tell me what I'm doing wrong ???
>
>class matrix
>{
>    float **m;
>    int   rows;
>    int   cols;
>public:
>
>            matrix(int,int);
>            ~matrix();
>    float*& operator[](int);
>    int     sizer();
>    int     sizec();
>    friend matrix 
>            operator*(matrix&, matrix&);
>};
>
  code deleted ...

The * function has to copy a matrix when it returns, but there is
no copy constructor.  The default copy constructor just copies the
contents, including pointers.  The segmentation fault probably occurs
when the destructors for the original (inside operator*) and the 
copy (cout << m * n) delete the same memory.  To fix this, add:

	matrix(const matrix&);  // Copy constructor
	matrix& operator=(const matrix&);  // Assignment

to your class.  Both functions should allocate new memory when they copy
their arguments.

Also, you may want to change operator* to:

	friend matrix operator*(const matrix&, const matrix&);

so you can pass temporary expressions, as in a * b * c.

-----------------------------------------
Matt Mahoney, 407-727-5431, mvm@epg.harris.com
#include <disclaimer.h>