Re: removing constness of a true const

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From	Ian Collins <ian-news@hotmail.com>
Newsgroups	comp.lang.c++
Subject	Re: removing constness of a true const
Date	Sat, 16 Jul 2011 09:30:16 +1200
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On 07/16/11 09:08 AM, Paavo Helde wrote:
> Victor Bazarov<v.bazarov@comcast.invalid>  wrote:
>>
>> Any data initializing which requires code to be executed at runtime
>> cannot be put in "the 'const' segment" - the segment would not be const,
>> would it?
>
> In principle the program can perform the dynamic initialisation of const
> objects at startup, then mark the relevant VM pages as read-only. I guess
> no implementation bothers to do such things currently.
>
> The most imminent danger with changing const objects is that the compiler
> is allowed to optimize away repeated reads of the data even if it cannot
> prove the data is not modified meanwhile. This might lead to subtle bugs,
> appearing probably only in optimized compilations. So even if the class
> object is not "truly const", it might effectively act as such.

A really simple example is:

void g( int );

int main()
{
   const int n = 42;
   int* p = const_cast<int*>(&n);
   *p = 43;
   g(n);
   g(*p);
}

-- 
Ian Collins

Thread

removing constness of a true const siddhu <siddharth.sng@gmail.com> - 2011-07-14 08:07 -0700
  Re: removing constness of a true const Victor Bazarov <v.bazarov@comcast.invalid> - 2011-07-14 11:55 -0400
    Re: removing constness of a true const siddhu <siddharth.sng@gmail.com> - 2011-07-14 09:25 -0700
      Re: removing constness of a true const Victor Bazarov <v.bazarov@comcast.invalid> - 2011-07-14 16:18 -0400
        Re: removing constness of a true const "Balog Pal" <pasa@lib.hu> - 2011-07-15 01:18 +0200
          Re: removing constness of a true const Victor Bazarov <v.bazarov@comcast.invalid> - 2011-07-15 08:17 -0400
            Re: removing constness of a true const "Balog Pal" <pasa@lib.hu> - 2011-07-15 18:04 +0200
              Re: removing constness of a true const Ian Collins <ian-news@hotmail.com> - 2011-07-16 08:36 +1200
                Re: removing constness of a true const "Balog Pal" <pasa@lib.hu> - 2011-07-16 02:00 +0200
            Re: removing constness of a true const Paavo Helde <myfirstname@osa.pri.ee> - 2011-07-15 16:08 -0500
              Re: removing constness of a true const Ian Collins <ian-news@hotmail.com> - 2011-07-16 09:30 +1200
  Re: removing constness of a true const "Balog Pal" <pasa@lib.hu> - 2011-07-14 19:05 +0200
    Re: removing constness of a true const siddhu <siddharth.sng@gmail.com> - 2011-07-14 12:05 -0700
      Re: removing constness of a true const "Bo Persson" <bop@gmb.dk> - 2011-07-14 23:33 +0200
        Re: removing constness of a true const Victor Bazarov <v.bazarov@comcast.invalid> - 2011-07-14 17:46 -0400
          Re: removing constness of a true const "Bo Persson" <bop@gmb.dk> - 2011-07-16 08:32 +0200
  Re: removing constness of a true const "Johannes Schaub (litb)" <schaub.johannes@googlemail.com> - 2011-07-15 15:02 +0200
    Re: removing constness of a true const Victor Bazarov <v.bazarov@comcast.invalid> - 2011-07-15 09:54 -0400

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