Path: utzoo!utgpu!water!watmath!clyde!att!whuts!mhuxh!mhuxu!mhuxt!mhuxi!mhuhk!mhuxo!ulysses!andante!alice!bs
From: bs@alice.UUCP (Bjarne Stroustrup)
Newsgroups: comp.lang.c++
Subject: Re: Friend specifier considered harmful
Summary: useful for operators
Message-ID: <8163@alice.UUCP>
Date: 3 Sep 88 14:32:30 GMT
References: <61@cybaswan.UUCP> <1988Sep2.174327.6439@ateng.uucp>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 37



A friend function can be used to express that coercions is acceptable
for the left hand operand of an operator. Such coercions are not aceptable
for the left hand operand of an operator defined as a member function:

	class Int {
		// ...
	public:
		Int(int);	// e.g. Int(27)
		Int(string);	// e.g. Int("123456789123456789")

		friend Int operator+(Int,Int);
		friend Int operator*(Int,Int);
		// ...
		Int operator=(Int);
		// ...
	};

	main()
	{
		Int a = 1;

		a+1;		// operator+(a,Int(1))
		1+a;		// operator+(Int(1),a)
		a = 1;		// a.operator+(Int(1))
		1 = a;		// illegal
	}

This could be expressed without the use of friend functions (but not
in C++, since you cannot specify a member function for a built-in type
so int::operator(Int) cannot be defined). It could, however, also be
expressed without the use of member functions.

It can also be expressed without the use of coercions, but that leads
to an explosion of operator functions.