Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!ut-emx!nowhere!sking
From: sking@nowhere.uucp (Steven King)
Newsgroups: comp.lang.c++
Subject: Re: friend operator +(l,r) vs. operator +(r)
Message-ID: <1990Nov29.023619.29838@nowhere.uucp>
Date: 29 Nov 90 02:36:19 GMT
Organization: nowhere
Lines: 53


   Various people have responded to various aspects of the original post; I
 think it is worth pointing out that under Cfront, for binary operators
 returning an instance of a class, one can put the body of the actual
 function in the corresponding assignment operator and define the binary
 operator as an inline that creates a temporary and assigns it the value
 of one of the binary args, then applies the assignment operator against
 the temporary and the remaining binary arg, returning the temporary.
   This DOES NOT result in a dangling reference, even if the translator can
 not inline the expression. ie.,

class point {
	double	x,y,z ;
public:
	point ( point & ) ;
	void operator += ( point & ) ;
	inline friend point &operator + ( point &, point & );
} ;

inline point &operator + ( point &a1, point &a2 )
{
	point a0 = a1 ;
	a0 += a2 ;
	return a0 ;
}

   This works, even in expressions like

	point p1, p2, p3, p4 ;

	p1 = p2 + p3 + p4 ;

 because the temporaries are created at the scope of the invoking expression.
 It will work even though the translator may not be able to inline the second
 invocation of the operator + ; it will create a static function taking three
 arguements, one of which is the temporary result.

   The biggest problem is that a lot of temporaries can be created, often
 without the programmer being aware of it, and the complexity may overwhelm
 less robust C compilers. 

	CAVEAT

   This behavior is easily demonstrated for Cfront 2.0, however I could not
 find anything in Lippman, E&S, or the Cfront manual to indicate if was
 one of those peculuarities specific to Cfront or part of the language.


-- 
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