Path: utzoo!attcan!uunet!wuarchive!husc6!ngo
From: ngo@tammy.harvard.edu (Tom Ngo)
Newsgroups: comp.lang.c++
Subject: [SUMMARY] Three ways to declare a function "const" in C++ 2.0
Message-ID: <NGO.90Oct16102625@tammy.harvard.edu>
Date: 16 Oct 90 14:26:25 GMT
Sender: news@husc6.harvard.edu
Distribution: comp
Organization: Harvard Chemistry Department
Lines: 62


Thanks to all those who replied to my (badly worded) query, posted in
comp.lang.c++: 

ngo> In g++, you can declare a function "const", e.g.
ngo>       const double sin( double x )
ngo> if the function examines nothing but its arguments and has no
ngo> side-effects.  (Such a function can be subject to common
ngo> subexpression elimination.)  Is this a part of C++ 2.0?  If not,
ngo> why not?

I say my query was badly worded because most of the replies
(understandably) described "const member functions", i.e. member
functions which (unless const is casted away) do not modify *this.
The syntax for defining such a function would go:

    double sin( double x ) const { /* ... */ }

My question really referred to the syntax in my original posting, i.e.

    const double sin( double x ) { /* ... */ }

AT&T and GNU C++ treat this syntax in different ways:

    [AT&T]  The "const" modifies the type "double", so the function
    returns a "const double", i.e. a double which cannot be modified.
    In this particular context, the "const" is meaningless--the value
    returned by a function to its caller can't be modified anyway.

    [GNU]  The "const" syntax is co-opted to mean something else.  It
    now declares something about the function itself:  that if the
    function is called twice, and the arguments are not altered
    between calls, the second call to the function can be eliminated
    without changing the meaning of the program.  This kind of const
    is used to eliminate redundant calculations in a sequence like

	double x, y, z;
	x = M_PI * 0.25;
	y = sin(x);
	z = sin(x);

    The compiler can replace this with

	double x, y, z;
	x = M_PI * 0.25;
	y = sin(x);
	z = y;

My opinion, for what it's worth, is that the G++ meaning is much more
useful.

Tony Hansen remarked:

    It's a relatively new extension of g++. I'm sure that it will be
    considered by the ANSI committee. I'm also sure that Bjarne
    Stroustrup has looked at the extensions that g++ provides, but I
    don't think any of them have made it back into C++ in the same
    fashion that g++ implemented them.
--
  Tom Ngo
  ngo@harvard.harvard.edu
  617/495-1768 lab number, leave message