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From: daniel@saturn.ucsc.edu (Daniel Edelson)
Newsgroups: comp.lang.c++
Subject: Re: operator []: ref vs value
Message-ID: <10844@saturn.ucsc.edu>
Date: 22 Feb 90 18:31:07 GMT
References: <5216@brazos.Rice.edu>
Reply-To: daniel@saturn.ucsc.edu (Daniel Edelson)
Organization: University of California, Santa Cruz
Lines: 45

In article <5216@brazos.Rice.edu> dougm@zamenhof.rice.edu (Doug Moore) writes:
>
>I can't seem to find a way to do precisely what I want; I want a
>sparse vector data type.  I can declare a float-returning operator []
>that returns 0.0 if the array index corresponds to a row for which no
>nonzero is stored.  I can declare a reference-to-float-returning
>operator[] that creates a new entry in my linked list of nonzero
>values when the row has no corresponding nonzero stored.
>Unfortunately, I want one behavior when the pseudo-array access occurs
>as an rvalue and another when it appears as an lvalue.  Can I get what
>I want?
>
>Dougm

You can have the [] operator return a dummy type and then overload
the assignment operator on the dummy type. This is not real efficient
but it seems to give correct semantics. I wrote this code for reference
counting vectors efficiently, but its not efficient. It should work for
sparse vectors well, though.

class vector;

struct dummy {		// A dummy refers to a specific vector element
	vector * vp;
	int index;
	operator int&()			{ /* return the element value */} 
	void operator=(int & t);
	dummy(vector *vvpp, int i) : vp(vvpp), index(i) { }
};

struct vec {
	// implementation details are yours

	dummy & operator[](int index) { return dummy(this, index); }
};

inline
void dummy::operator=(int & t) 
{ 
	// Do whatever necessary to assign t to the vector 
	// component at index in *vp.
}

daniel@cis.ucsc.edu
daniel edelson