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From: jbuck@galileo.berkeley.edu (Joe Buck)
Newsgroups: comp.lang.c++
Subject: Re: why is this program slow?
Message-ID: <10214@pasteur.Berkeley.EDU>
Date: 15 Jan 91 18:43:24 GMT
References: <1991Jan9.002244.23398@news.cs.indiana.edu> <1738@rose.ACA.MCC.COM> <1877@ruunsa.fys.ruu.nl>
Sender: news@pasteur.Berkeley.EDU
Reply-To: jbuck@galileo.berkeley.edu (Joe Buck)
Lines: 36

In article <1877@ruunsa.fys.ruu.nl>, muts@fysaj.fys.ruu.nl (Peter Mutsaers /1000000) writes:
|> It is a pity that the c++ optimizer does not (yet) automatically
|> change a = a + b into a+=b if operator+= is defined too. It would
|> reduce however the freedom to use operators like += *= etc.

I got this trick from Andy Koenig.

Whenever I write a class with arithmetic operators, I always write
+=, *=, etc, as member functions and then write (let's say the class
is Foo)

inline Foo operator+ (const Foo& a, const Foo& b) {
	Foo tmp(a);
	tmp += b;
	return tmp;
}

Notice that this function does not need to be a friend (so when
people ask me whether arithmetic operators should be members or
friends, I reply, neither).  With this kind of structure, most
optimizers can convert

func (Foo& x, Foo& y) {
	...

	Foo c = x + y;
	...
}

into
	Foo c(x); c += y;

with little trouble.
--
Joe Buck
jbuck@galileo.berkeley.edu	 {uunet,ucbvax}!galileo.berkeley.edu!jbuck