MESSAGE
| DATE | 2008-09-03 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [NYLXS - HANGOUT] C++ Workshop Const Functions
|
Date: Sun, 31 Aug 2008 02:28:47 -0500
From: Paavo Helde
Lines: 61
Message-ID:
NNTP-Posting-Date: Sun, 31 Aug 2008 02:28:47 -0500
Newsgroups: comp.lang.c++
Path:
reader1.panix.com!panix!bloom-beacon.mit.edu!newsswitch.lcs.mit.edu!newsfeed.cwix.com!dc3peer2.nntp.savvis.net!peer.nntp.savvis.net!cycny02.gnilink.net!cyclone1.gnilink.net!gnilink.net!nx01.iad01.newshosting.com!newshosting.com!216.196.98.140.MISMATCH!Xl.tags.giganews.com!border1.nntp.dca.giganews.com!nntp.giganews.com!local02.nntp.dca.giganews.com!news.giganews.com.POSTED!not-for-mail
References:
Subject: Re: error: passing `const Weight' as `this'
argument of `float Weight::wgt()' discards qualifiers
User-Agent: Xnews/5.04.25
X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers
X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your
complaint properly
X-Complaints-To: abuse-at-giganews.com
X-DMCA-Notifications: http://www.giganews.com/info/dmca.html
X-Postfilter: 1.3.39
X-Trace:
sv3-zSNH/CfU/RfOVKi6IK9qk51Y6F54YSOTRXqItMKO02dvVAoMeeeBQWpqroTC/2K3Wc8R77P6irb4kMd!dwiE/BMzw9Bb9OCwBI6nrMTLMlwlglOTNgk5Dy0sS5a1LZ042vB4rt5576MFUFxj38Ake7Vz
X-Usenet-Provider: http://www.giganews.com
Xref: panix comp.lang.c++:1019775
MIME-Version: 1.0
Content-Type: text/plain
Ruben kirjutas:
> On Sat, 30 Aug 2008 01:39:48 -0500, Paavo Helde wrote:
>
>>> Weight& Weight::operator=(const Weight &fweight) {
>>> wgt_ = fweight.wgt();
>>
>> And here you call it through a const Weight reference.
>>
>> Make your getter methods const member functions.
>
> what does making the fuction const do practically? If I make a const
> function that assings some member a value, will it compile?
No, it will not, and that's the point of the const system. It holds your
hand and tries to prevent you shoot in your leg.
Some people argue that constness it not the only or most important
aspect
to keep in mind when working with objects, and there should be no
obligation to use it. However, it has been built into the language (in
signatures of assignment op and copy ctor and the rule about binding
temporaries to const references) as well as into the standard library,
so
practically there is no alternative to making all code const-correct
from
the start.
Here, the standard signature for assignment op is as you wrote:
Weight& Weight::operator=(const Weight &fweight);
The constness of the argument now propagates further to every use of the
fweight reference. I try to explain the propagation mechanism in more
detail, feel free to use your books instead if I am not making any sense
;-)
The signature promises that inside this function the fweight argument is
kept const. Now you are calling function fweight.wgt() which is
non-const
and thus may potentially modify the object referenced by fweight. Thus
the
compiler cannot be sure it can keep up with the promises of operator= ()
signature, so the compilation is aborted.
Now, if you make wgt() const,
class Weight {
public:
float wgt() const { return wgt_; };
Now the operator=() is happy and compiles. OTOH, if you had called some
non-const member function on the same object from wgt(), you would have
got another error there, i.e. the error will also "propagate". Now you
could add 'const' to this next member function and see if the thing
compiles, etc.
However, making the code const-correct this way is quite tedious, better
is to decide up-front which member functions should not modify the
object,
and mark them 'const' from the start. The "getter" functions like wgt()
are the obvious candidates.
hth
Paavo
--
http://www.mrbrklyn.com - Interesting Stuff
http://www.nylxs.com - Leadership Development in Free Software
So many immigrant groups have swept through our town that Brooklyn, like Atlantis, reaches mythological proportions in the mind of the world - RI Safir 1998
http://fairuse.nylxs.com DRM is THEFT - We are the STAKEHOLDERS - RI Safir 2002
"Yeah - I write Free Software...so SUE ME"
"The tremendous problem we face is that we are becoming sharecroppers to our own cultural heritage -- we need the ability to participate in our own society."
"> I'm an engineer. I choose the best tool for the job, politics be damned.<
You must be a stupid engineer then, because politcs and technology have been attached at the hip since the 1st dynasty in Ancient Egypt. I guess you missed that one."
© Copyright for the Digital Millennium
|
|
