Path: utzoo!attcan!uunet!husc6!uwvax!umn-d-ub!nic.MR.NET!hal!cwjcc!tut.cis.ohio-state.edu!osu-cis!att!ulysses!andante!alice!bs
From: bs@alice.UUCP (Bjarne Stroustrup)
Newsgroups: comp.lang.c++
Subject: Re: Continuing efforts to build a CM++ (sic) environment (long) ...
Summary: a technique
Keywords: compile time attributes, expressions, operators, temporaries
Message-ID: <8415@alice.UUCP>
Date: 11 Nov 88 17:45:29 GMT
References: <17796@shemp.CS.UCLA.EDU>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 29



Giving control to the (library) programmer to the point of controlling
temporary variable usage is non-trivial. However, there are ways of using
C++ so that temporaries isn't user.

Assume a class Matrix for which creation/copying/destruction is so expensive
that you'd rather not have temporary variables introduced:

	f() {
		Matrix a,b,c;
		// ...
		a = b + c;	// yuck: temporary used
		a = a * c;	// yuck: temporary used
	}

however:

	f()
	{
		Matrix b,c;
		// ...
		Matrix a = b + c;	// temporary not used
		a *= c;			// temporary not used
	}

Initialization is typically cheaper than assignment (since the compiler
doesn't need to introduce a temporary to protect against aliasing) and
*=, +=, etc. is almost always a win for user-defined types.