Xref: utzoo comp.lang.c++:12053 comp.std.c++:692
Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!uunet!motcid!stephens
From: stephens@motcid.UUCP (Kurt Stephens)
Newsgroups: comp.lang.c++,comp.std.c++
Subject: Re: distinguishing operator[] on left and right
Message-ID: <6657@celery34.UUCP>
Date: 5 Mar 91 16:27:48 GMT
References: <1991Feb28.212419.20920@ndl.com> <1991Mar2.000705.3496@mathcs.sjsu.edu> <11634@pasteur.Berkeley.EDU> <1991Mar4.211036.9508@am.dsir.govt.nz>
Organization: Motorola Inc., Cellular Infrastructure Div., Arlington Heights, IL
Lines: 55

robert@am.dsir.govt.nz (Robert Davies) writes:
>Currently (in Turbo C++ or Glockenspiel C++; my version of Zortech can't
>distinguish between the 2 versions) you need to write

>      char c = ((const string)g)[3];

>to get the second version. Which is a bit messy.

>But wouldn't it be reasonable for the second version to be the default if the
>compiler can tell that the operation won't affect the value of g?  Was this
>kind of issue raised in the recent discussion on const in comp.std.c++?

	How could the compiler tell that the operation won't affect the
value of g? the string::operator[]() could be doing just about anything
to the privates of g.  C++ compliers cannot read minds,
or understand the internal semantics of any functions.

Example:
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-==-=-=-=-=-=-=-=-=-
#include	<stdio.h>

class X {
	int	i;
public:
	X( int I ) : i(I) {}
	int	operator[](int I) { return i = I; }
	void	print() { printf("%d\n", i ); }
};

main() {
	X	a = 1;
	a.print();
	a[-1];
	a.print();
}
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

X::operator[](int) means "assign to member i", but because of the
association with C's operator[], most container classes limit operator[]()
semantics to be "return a lvalue", "return a rvalue" or "lookup in table",
which is great because it makes for readable code.

Obviously, X::operator[](int) is a bad choice for "assign to member i";
X::operator=(int) would have been much more intuitive handle.

But we (and the complier) cannot assume that operator[]() never
modifies the state of the class or its instances.

Kurt A. Stephens		Foo::Foo(){return Foo();}
stephens@void.rtsg.mot.com	"When in doubt, recurse."

-- 

Kurt A. Stephens		Foo::Foo(){return Foo();}
stephens@void.rtsg.mot.com	"When in doubt, recurse."