Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!hao!oddjob!gargoyle!ihnp4!homxb!mtuxo!mtgzz!drutx!druhi!weh
From: weh@druhi.ATT.COM (HopkinsBWE)
Newsgroups: comp.lang.c++
Subject: Re: Now a bug (?)
Message-ID: <2512@druhi.ATT.COM>
Date: 29 Dec 87 17:39:48 GMT
References: <370@targon.UUCP>
Organization: AT&T, Denver, CO
Lines: 64
Summary: formal argument needs to be a const &

In article <370@targon.UUCP>, wim@targon.UUCP (Wim C. J. van Eerdt) writes:
> #include <stream.h>
> 
> class	string  {
> 	//...
>    public:
> 
> 	string();
> 	string( char*   );
> 	string( string& );
> 
> 	friend ostream& operator<< ( ostream&, string& );
> 
> } ;
> 
> const string	text = "text";
> 
> void	call()
> {
> 
> 	cout << text;	// reference to a const object.
> 
> }
> 
> 
> 	A colleague typed the exhibited program and found an error on line
> 		cout << text;
> The message is correct: the 2nd argument of operator<< is a reference to a
> string. So he changed the definition into
> 		friend ostream& operator<< ( ostream&, string );
> Surprise, that did not work either: the constructor string( string& ) still
> takes the reference of the constant object. Neither skipping the constructor
> nor skipping the cont declaration is the answer, who knows the answer.

Cfront won't allow one to pass a constant object by reference into a
function unless the formal argument is also a constant. The error message from
cfront (from a copy of the example in a file named ctest.c) is:

CC  ctest.c:
"ctest.c", line 18: error:  reference to const object
1 error

Line 18 in my file is the ``cout << text; '' line.

Thus, the function should have been declared as:

 	friend ostream& operator<< ( ostream&, const string& );

Sure enough, cfront happily compiled it after the change.

Since changes to parameters passed by reference change the actual
argument, cfront cannot allow a constant to be passed as a non-constant
reference.

The solution of declaring the argument as simply a ``string'' fails
because that requires the creation of a new copy of ``string'' from an
existing string, meaning that string::string(string&) must be called. Since
it declares the argument as a nonconstant reference, it suffers from the
same problem as with operator>> (the same error message from cfront). The
solution is similar: declare the constructor as ``string(const string&);''.
This solution, too, has been confirmed by cfront.

			Bill Hopkins
			AT&T
			ihnp4!druhi!weh