Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!microsoft!jimad
From: jimad@microsoft.UUCP (Jim ADCOCK)
Newsgroups: comp.lang.c++
Subject: Re: operator []: ref vs value
Message-ID: <51111@microsoft.UUCP>
Date: 28 Feb 90 19:28:12 GMT
References: <5216@brazos.Rice.edu>
Reply-To: jimad@microsoft.UUCP (Jim ADCOCK)
Organization: Microsoft Corp., Redmond WA
Lines: 101

/****
You can't make two versions of op[] for lvalue and rvalue, but you can
differentiate the situations where an op[] is taken, and where an op[] is
taken and value is changed.  The general trick is for op[] to return a 
smart proxy for that element, and a special assignment to the proxy is
defined that updates the vector if the value of the proxy, [and thus the
element] changes.  Just to confuse things, in the below sketchy example,
I use the same class "SVEL" for both the elements of the vector, and the
proxy to that element.
****/

class SVEL// a sparse vector element
{
	SVEL* psvel;
	class SV& sv; 
	int i;
	double d;
public:
	SVEL(SV& svT, int iT, SVEL* psvelT, double dT=0.0);
	SVEL(SV& svT, int iT);
	SVEL& operator=(double d);
	void Attach(SVEL* psvelT) { psvel = psvelT; }
	SVEL* Next() { return psvel; }
	int Index() { return i; }
	void Print();
	operator double() { return d; }
};
SVEL::SVEL(SV& svT, int iT, SVEL* psvelT, double dT) : 
	sv(svT), i(iT), psvel(psvelT), d(dT) {}

SVEL::SVEL(SV& svT, int iT) : 
	sv(svT), i(iT), psvel(0), d(0) {}

void SVEL::Print() { printf("(%d, ",i); printf("%g) ",d); }

class SV	// a sparse vector
{
	SVEL* psvel;
public:
	SV();
	void Attach(SVEL* psvelT) 
	{ SVEL* pT = psvel; psvel = psvelT; psvel->Attach(pT); }
	SVEL& operator[](int i);
	void Print();
};
SV::SV() : psvel(0) {}

SVEL& SVEL::operator=(double dT)
{
	if ((d==0.0) && (dT!=0.0))
	{
		d = dT;
		sv.Attach(this);
	}
	else d = dT;

	return *this;
}

void SV::Print()
{
	SVEL* p = psvel;
	while (p)
	{
		p->Print();
		p = p->Next();
	}
	putchar('\n');
}

SVEL& SV::operator[](int i)
{
	SVEL* p = psvel;
	while (p && (i != p->Index()))
		p = p->Next();

	if (p)
		return *p;
	else
		return *new SVEL(*this, i);
}

int main()
{
	SV v1, v2;
	v1[100] = 10.0;
	v1[200] = 20.0;
	v2[10] = v1[1];
	v2[20] = v1[200];
	v2[30] = 300.0;
	v1.Print();
	v2.Print();
	v2[20] = 200.0;
	v2.Print();
	double d20 = v2[20];
	printf("%g\n", d20);

	return 0;
}

// gc left as an exercise :-)