Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!gatech!purdue!decwrl!vixie!avsd!daniel
From: daniel@avsd.UUCP (Daniel Edelson)
Newsgroups: comp.lang.c++
Subject: Re: const member functions ?
Message-ID: <1771@avsd.UUCP>
Date: 18 Jul 89 22:40:44 GMT
References: <6590205@hplsla.HP.COM> <9633@alice.UUCP>
Reply-To: daniel@avsd.UUCP (Daniel Edelson)
Organization: AMPEX Corporation, Redwood City, CA and UC Santa Cruz
Lines: 52

>In article <6590205@hplsla.HP.COM>, jima@hplsla.HP.COM (Jim Adcock) writes:
>
>> Is there a way to declare a member function const -- IE to tell the compiler
>> that the member function does not change the value of this nor *this

In article <9633@alice.UUCP> ark@alice.UUCP (Andrew Koenig) responds:
>
>Yes [in 2.0]:
>
>	class foo {
>		int n;
>	public:
>		void setsize(int k) { n = k; }
>		int getsize() const { return n; }
>	};
>
>In a member function defined with `const' this way, `this' is
>a pointer to a constant object.
>-- 
>				--Andrew Koenig
>				  ark@europa.att.com


Why is this feature useful? Is it to improve code readability
by making it clear when a member function does not alter any 
member data? Is there also a reliability motivation?
RE: jima's other question, is there also a syntax for specifying 
that `this' is a constant pointer rather than a pointer to a 
constant object?

How about this?

struct S {
	int x;
	void cmf() const;
};

void put(S *p) { cin >> p->x; }
void look(const S *p) { cout << p->x; }

void S::cmf() const
{
    put(this);  // illegal, put() takes a "S *" but `this' is a "const S *"
    look(this); // legal, look() takes a "const S *", which `this' is
    this=this;  // legal, `this' is of type "const S *", not "S *const"
}

Is this correct?

Daniel Edelson
Currently:  daniel@avsd.UUCP        (Ampex Corporation, Redwood City, CA)
And also:	daniel@saturn.ucsc.edu  (Univ. of Calif., Santa Cruz)