Path: utzoo!attcan!uunet!microsoft!jimad
From: jimad@microsoft.UUCP (Jim ADCOCK)
Newsgroups: comp.lang.c++
Subject: Re: Extensions to []
Message-ID: <55101@microsoft.UUCP>
Date: 8 Jun 90 18:50:36 GMT
References: <1589@ole.UUCP>
Reply-To: jimad@microsoft.UUCP (Jim ADCOCK)
Organization: Microsoft Corp., Redmond WA
Lines: 109

In article <1589@ole.UUCP> powell@ole.UUCP (Gary Powell) writes:
>I am new to C++ but after a seminar and some play time I tried writting
>a class for two diminsional arrays.  What I found myself wanting was
>a overloaded operator [][](int)(int).  However my GNU g++ 1.35 compiler
>complains bitterly when I tried to code this.  Why not use the (int,int)
>operator? Well I was using that for the initialization. It would seem to
>me that since C allows array[][] access that C++ should allow overloading
>of this also.
>
>With this background has there been any discussion on the overloading of
>[][] ....[]?
>
>And is this a reasonable extension to the language?

No, it shouldn't be necessary, and [][] is not one operator but two.  Changing
[][] from two operators to one would significantly change C++ from the C past.
And doing so shouldn't be necessary.

There are two common tricks used by people doing 2-dim [or n-dim] arrays
in C++.  Examples follow.  The first trick is to build up your objects
from rows and columns as the two [] operators implies.  The second trick is
to return a proxy object from the first application of [], that "remembers"
where you are in the 2-dim [n-dim] array after the first application, and
then the second application of [] is applied to the proxy object, which 
"remembers" the previous index, and thus has all the information necessary
to return the correct element in the 2-dim [n-dim] array.  BUT: be forwarned
there are a fair amount of C and/or C++ compilers out there that have a 
hard time correctly compiling the code necessary to do proxies!


// Example of building up a two dimemsional array one dimension at a time:


extern "C"
{
#include "stdio.h"
}


class COL
{
public:
	int c[10];
	int& operator[](int i) { printf("COL[%d] ",i); return c[i]; }
};

class MAT
{
public:
	COL r[10];
	COL& operator[](int j) { printf("ROW[%d] ",j); return r[j]; }
};

main()
{
	MAT mat;

	mat[5][6] = 56;
	printf("\n");
	mat[6][5] = 65;
	printf("\n");

	printf("mat[5][6] %d\n", mat[5][6]);
	printf("mat[6][5] %d\n", mat[6][5]);
}

// example of using proxies:


extern "C"
{
#include "stdio.h"
}

#include "shortnew.h"

typedef int* PI;

class PROXY
{
	const PI c;
public:
	PROXY(const PI cc) : c(cc) {}
	int& operator[](int i) const { printf("COL[%d] ",i); return c[i]; }
};

class MAT
{
	int i[100];
public:
	PROXY operator[](int j) const 
	{ 
		printf("ROW[%d] ",i); 
		return &(i[j*10]); 
	}
};

main()
{
	MAT mat;

	mat[5][6] = 56;
	printf("\n");
	mat[6][5] = 65;
	printf("\n");

	printf("mat[5][6] %d\n", mat[5][6]);
	printf("mat[6][5] %d\n", mat[6][5]);
}