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What a pointer really points to

 
 
Steven T. Hatton
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      12-09-2006
I found the example code below, listed in the book described here:
http://cartan.cas.suffolk.edu/moin/OopDocbookWiki

The result was a bit surprising. I guess it falls into the category
of "that's what you get for lying". Can the behavior demonstrated be
explained in standardese? Yes, I know the exact result is "undefined
behavior". I can see what happened. But what was the actual violation?

// Miles are converted to kilometers.
#include <QTextStream>

QTextStream cin(stdin, QIODevice::ReadOnly);
QTextStream cout(stdout, QIODevice::WriteOnly);
QTextStream cerr(stderr, QIODevice::WriteOnly);

const double m2k = 1.609; // conversion constant

inline double mi2km(int miles) {
return (miles * m2k);
}

int main() {
int miles;
double kilometers;
cout << "Enter distance in miles: " << flush;
cin >> miles ;
kilometers = mi2km(miles);
cout << "This is approximately "
<< static_cast<int>(kilometers)
<< "km."<< endl;
cout << "Without the cast, kilometers = "
<< kilometers << endl;
double* dp = const_cast<double*>(&m2k);
cout << "m2k: " << m2k << endl;
cout << "&m2k: " << &m2k << " dp: " << dp << endl;
cout << "*dp: " << *dp << endl;
*dp = 1.892; /* What are we attempting to do here?*/
cout << "Can we reach this statement? " << endl;
return 0;
}

/*OUT
Enter distance in miles: 23
This is approximately 37km.
Without the cast, kilometers = 37.007
m2k: 1.609
&m2k: 0x8049048 dp: 0x8049048
*dp: 1.609
Segmentation fault
*/


--
NOUN:1. Money or property bequeathed to another by will. 2. Something handed
down from an ancestor or a predecessor or from the past: a legacy of
religious freedom. ETYMOLOGY: MidE legacie, office of a deputy, from OF,
from ML legatia, from L legare, to depute, bequeath. www.bartleby.com/61/
 
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kwikius
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Posts: n/a
 
      12-09-2006
Steven T. Hatton wrote:
> I found the example code below, listed in the book described here:
> http://cartan.cas.suffolk.edu/moin/OopDocbookWiki


Well that just goes to show you don't it

#include <quan/out/length.hpp>

int main() {


std::cout << "Enter distance in miles: ";
quan::length::mi miles;
std::cin >> miles.reference_numeric_value<quan::length::mi>(); ;
quan::length::km kilometers = miles;
std::cout << "\nThat is approximately " << kilometers << "\n\n";

std::cout << "Can we reach this statement?\n\n";
std::cout << "Sure! ... Feel the Power of The Quan \n\n";
std::cout << "-------------------------------------\n";
std::cout << "Quan documentation :\n\n";
std::cout << "<http://quan.sourceforge.net>\n\n";
std::cout << "Quan download and cvs :\n\n";
std::cout << "<http://sourceforge.net/projects/quan>\n\n";
std::cout << "-------------------------------------\n";
return 0;
}
/*
output:
Enter distance in miles: 2

That is approximately 3.21869 km

Can we reach this statement?

Sure! ... Feel the Power of The Quan

-------------------------------------
Quan documentation :

<http://quan.sourceforge.net>

Quan download and cvs :

<http://sourceforge.net/projects/quan>

-------------------------------------
*/

 
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John Carson
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Posts: n/a
 
      12-09-2006
"Steven T. Hatton" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)
> I found the example code below, listed in the book described here:
> http://cartan.cas.suffolk.edu/moin/OopDocbookWiki
>
> The result was a bit surprising. I guess it falls into the category
> of "that's what you get for lying". Can the behavior demonstrated be
> explained in standardese? Yes, I know the exact result is "undefined
> behavior". I can see what happened. But what was the actual
> violation?
>
> // Miles are converted to kilometers.
> #include <QTextStream>
>
> QTextStream cin(stdin, QIODevice::ReadOnly);
> QTextStream cout(stdout, QIODevice::WriteOnly);
> QTextStream cerr(stderr, QIODevice::WriteOnly);
>
> const double m2k = 1.609; // conversion constant
>
> inline double mi2km(int miles) {
> return (miles * m2k);
> }
>
> int main() {
> int miles;
> double kilometers;
> cout << "Enter distance in miles: " << flush;
> cin >> miles ;
> kilometers = mi2km(miles);
> cout << "This is approximately "
> << static_cast<int>(kilometers)
> << "km."<< endl;
> cout << "Without the cast, kilometers = "
> << kilometers << endl;
> double* dp = const_cast<double*>(&m2k);
> cout << "m2k: " << m2k << endl;
> cout << "&m2k: " << &m2k << " dp: " << dp << endl;
> cout << "*dp: " << *dp << endl;
> *dp = 1.892; /* What are we attempting to do here?*/
> cout << "Can we reach this statement? " << endl;
> return 0;
> }
>
> /*OUT
> Enter distance in miles: 23
> This is approximately 37km.
> Without the cast, kilometers = 37.007
> m2k: 1.609
> &m2k: 0x8049048 dp: 0x8049048
> *dp: 1.609
> Segmentation fault
> */


Deleting the irrelevant, you have:

const double m2k = 1.609;

int main()
{
double* dp = const_cast<double*>(&m2k);
*dp = 1.892;
return 0;
}

Thus dp points to a const variable and you attempt to change that const
variable using a dereferenced dp.

The operating system has presumably stored m2k in a read-only section of
memory, so raises a segmentation fault when you attempt to write to that
memory.

--
John Carson


 
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Steven T. Hatton
Guest
Posts: n/a
 
      12-09-2006
John Carson wrote:

> "Steven T. Hatton" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)
>> I found the example code below, listed in the book described here:
>> http://cartan.cas.suffolk.edu/moin/OopDocbookWiki
>>
>> The result was a bit surprising. I guess it falls into the category
>> of "that's what you get for lying". Can the behavior demonstrated be
>> explained in standardese? Yes, I know the exact result is "undefined
>> behavior". I can see what happened. But what was the actual
>> violation?
>>
>> // Miles are converted to kilometers.
>> #include <QTextStream>
>>
>> QTextStream cin(stdin, QIODevice::ReadOnly);
>> QTextStream cout(stdout, QIODevice::WriteOnly);
>> QTextStream cerr(stderr, QIODevice::WriteOnly);
>>
>> const double m2k = 1.609; // conversion constant
>>
>> inline double mi2km(int miles) {
>> return (miles * m2k);
>> }
>>
>> int main() {
>> int miles;
>> double kilometers;
>> cout << "Enter distance in miles: " << flush;
>> cin >> miles ;
>> kilometers = mi2km(miles);
>> cout << "This is approximately "
>> << static_cast<int>(kilometers)
>> << "km."<< endl;
>> cout << "Without the cast, kilometers = "
>> << kilometers << endl;
>> double* dp = const_cast<double*>(&m2k);
>> cout << "m2k: " << m2k << endl;
>> cout << "&m2k: " << &m2k << " dp: " << dp << endl;
>> cout << "*dp: " << *dp << endl;
>> *dp = 1.892; /* What are we attempting to do here?*/
>> cout << "Can we reach this statement? " << endl;
>> return 0;
>> }
>>
>> /*OUT
>> Enter distance in miles: 23
>> This is approximately 37km.
>> Without the cast, kilometers = 37.007
>> m2k: 1.609
>> &m2k: 0x8049048 dp: 0x8049048
>> *dp: 1.609
>> Segmentation fault
>> */

>
> Deleting the irrelevant, you have:


It's often nice to have something that actually compiles and runs.
Compiling the code using the Standard Library would take a trivial amount
of effort.

> const double m2k = 1.609;
>
> int main()
> {
> double* dp = const_cast<double*>(&m2k);
> *dp = 1.892;
> return 0;
> }
>
> Thus dp points to a const variable


What does the Standard specify it should point to, or is that specified?

> and you attempt to change that const
> variable using a dereferenced dp.
>
> The operating system has presumably stored m2k in a read-only section of
> memory, so raises a segmentation fault when you attempt to write to that
> memory.


As I sated, I understand what happened. I'm just not sure what rule was
violated. I guess I go back to slogging my way through the artful prose of
the formal arcana of the Standard.

--
NOUN:1. Money or property bequeathed to another by will. 2. Something handed
down from an ancestor or a predecessor or from the past: a legacy of
religious freedom. ETYMOLOGY: MidE legacie, office of a deputy, from OF,
from ML legatia, from L legare, to depute, bequeath. www.bartleby.com/61/
 
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kwikius
Guest
Posts: n/a
 
      12-09-2006

Steven T. Hatton wrote:

> What does the Standard specify it should point to, or is that specified?


The only time you can use const_cast on some object reference is when
you *know* that the real object is not const. Any const cast on a
physically const object results in undefined behaviour. The better
option on stuff you *own* is to use mutable: That is my understanding
anyway:

struct my{
private:
int n;
public:
my():n(1),y(2){}
// mod is declared as a const function
// but maybe want to keep internal count
void mod1()const
{
// dodgy If someone declares a const my
my& mm = const_cast<my&>(*this);
++mm.n;
}
private:
mutable int y;
public:
void mod2()const
{ // always ok
++y;
}
};

int main()
{
my x;
x.mod1();
x.mod2();
}

regards
Andy Little

 
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kwikius
Guest
Posts: n/a
 
      12-09-2006
Steven T. Hatton wrote:

> What does the Standard specify it should point to, or is that specified?


The only time you can use const_cast on some object reference is when
you *know* that the real object is not const. Any const cast on a
physically const object results in undefined behaviour. The better
option on stuff you *own* is to use mutable: That is my understanding
anyway:

struct my{
private:
int n;
public:
my():n(1),y(2){}
// mod is declared as a const function
// but maybe want to keep internal count
void mod1()const
{
// dodgy If someone declares a const my
my& mm = const_cast<my&>(*this);
++mm.n;
}
private:
mutable int y;
public:
void mod2()const
{ // always ok
++y;
}
};

int main()
{
my x;
x.mod1();
x.mod2();
}

regards
Andy Little

 
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Kai-Uwe Bux
Guest
Posts: n/a
 
      12-09-2006
Steven T. Hatton wrote:

> John Carson wrote:
>
>> "Steven T. Hatton" <(E-Mail Removed)> wrote in message
>> news:(E-Mail Removed)
>>> I found the example code below, listed in the book described here:
>>> http://cartan.cas.suffolk.edu/moin/OopDocbookWiki
>>>
>>> The result was a bit surprising. I guess it falls into the category
>>> of "that's what you get for lying". Can the behavior demonstrated be
>>> explained in standardese? Yes, I know the exact result is "undefined
>>> behavior". I can see what happened. But what was the actual
>>> violation?
>>>
>>> // Miles are converted to kilometers.
>>> #include <QTextStream>
>>>
>>> QTextStream cin(stdin, QIODevice::ReadOnly);
>>> QTextStream cout(stdout, QIODevice::WriteOnly);
>>> QTextStream cerr(stderr, QIODevice::WriteOnly);
>>>
>>> const double m2k = 1.609; // conversion constant
>>>
>>> inline double mi2km(int miles) {
>>> return (miles * m2k);
>>> }
>>>
>>> int main() {
>>> int miles;
>>> double kilometers;
>>> cout << "Enter distance in miles: " << flush;
>>> cin >> miles ;
>>> kilometers = mi2km(miles);
>>> cout << "This is approximately "
>>> << static_cast<int>(kilometers)
>>> << "km."<< endl;
>>> cout << "Without the cast, kilometers = "
>>> << kilometers << endl;
>>> double* dp = const_cast<double*>(&m2k);
>>> cout << "m2k: " << m2k << endl;
>>> cout << "&m2k: " << &m2k << " dp: " << dp << endl;
>>> cout << "*dp: " << *dp << endl;
>>> *dp = 1.892; /* What are we attempting to do here?*/
>>> cout << "Can we reach this statement? " << endl;
>>> return 0;
>>> }
>>>
>>> /*OUT
>>> Enter distance in miles: 23
>>> This is approximately 37km.
>>> Without the cast, kilometers = 37.007
>>> m2k: 1.609
>>> &m2k: 0x8049048 dp: 0x8049048
>>> *dp: 1.609
>>> Segmentation fault
>>> */

>>
>> Deleting the irrelevant, you have:

>
> It's often nice to have something that actually compiles and runs.
> Compiling the code using the Standard Library would take a trivial amount
> of effort.
>
>> const double m2k = 1.609;
>>
>> int main()
>> {
>> double* dp = const_cast<double*>(&m2k);
>> *dp = 1.892;
>> return 0;
>> }
>>
>> Thus dp points to a const variable

>
> What does the Standard specify it should point to, or is that specified?


It is specified in [5.2.11/3].


>> and you attempt to change that const
>> variable using a dereferenced dp.
>>
>> The operating system has presumably stored m2k in a read-only section of
>> memory, so raises a segmentation fault when you attempt to write to that
>> memory.

>
> As I sated, I understand what happened. I'm just not sure what rule was
> violated.


The code has undefined behavior according to [7.1.5.1/4].


> I guess I go back to slogging my way through the artful prose
> of the formal arcana of the Standard.



Best

Kai-Uwe Bux
 
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=?ISO-8859-1?Q?Erik_Wikstr=F6m?=
Guest
Posts: n/a
 
      12-09-2006
On 2006-12-09 14:14, Steven T. Hatton wrote:
> John Carson wrote:
>
>> "Steven T. Hatton" <(E-Mail Removed)> wrote in message
>> news:(E-Mail Removed)
>>> I found the example code below, listed in the book described here:
>>> http://cartan.cas.suffolk.edu/moin/OopDocbookWiki
>>>
>>> The result was a bit surprising. I guess it falls into the category
>>> of "that's what you get for lying". Can the behavior demonstrated be
>>> explained in standardese? Yes, I know the exact result is "undefined
>>> behavior". I can see what happened. But what was the actual
>>> violation?
>>>
>>> // Miles are converted to kilometers.
>>> #include <QTextStream>
>>>
>>> QTextStream cin(stdin, QIODevice::ReadOnly);
>>> QTextStream cout(stdout, QIODevice::WriteOnly);
>>> QTextStream cerr(stderr, QIODevice::WriteOnly);
>>>
>>> const double m2k = 1.609; // conversion constant
>>>
>>> inline double mi2km(int miles) {
>>> return (miles * m2k);
>>> }
>>>
>>> int main() {
>>> int miles;
>>> double kilometers;
>>> cout << "Enter distance in miles: " << flush;
>>> cin >> miles ;
>>> kilometers = mi2km(miles);
>>> cout << "This is approximately "
>>> << static_cast<int>(kilometers)
>>> << "km."<< endl;
>>> cout << "Without the cast, kilometers = "
>>> << kilometers << endl;
>>> double* dp = const_cast<double*>(&m2k);
>>> cout << "m2k: " << m2k << endl;
>>> cout << "&m2k: " << &m2k << " dp: " << dp << endl;
>>> cout << "*dp: " << *dp << endl;
>>> *dp = 1.892; /* What are we attempting to do here?*/
>>> cout << "Can we reach this statement? " << endl;
>>> return 0;
>>> }
>>>
>>> /*OUT
>>> Enter distance in miles: 23
>>> This is approximately 37km.
>>> Without the cast, kilometers = 37.007
>>> m2k: 1.609
>>> &m2k: 0x8049048 dp: 0x8049048
>>> *dp: 1.609
>>> Segmentation fault
>>> */

>>
>> Deleting the irrelevant, you have:

>
> It's often nice to have something that actually compiles and runs.
> Compiling the code using the Standard Library would take a trivial amount
> of effort.
>
>> const double m2k = 1.609;
>>
>> int main()
>> {
>> double* dp = const_cast<double*>(&m2k);
>> *dp = 1.892;
>> return 0;
>> }
>>
>> Thus dp points to a const variable

>
> What does the Standard specify it should point to, or is that specified?


It should point to the const double m2k.

>> and you attempt to change that const
>> variable using a dereferenced dp.
>>
>> The operating system has presumably stored m2k in a read-only section of
>> memory, so raises a segmentation fault when you attempt to write to that
>> memory.

>
> As I sated, I understand what happened. I'm just not sure what rule was
> violated. I guess I go back to slogging my way through the artful prose of
> the formal arcana of the Standard.


The violation is trying to change the value of a const. On your system
this happens to result in a segmentation fault, but there are systems
and situations where it won't.

--
Erik Wikström
 
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Steven T. Hatton
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Posts: n/a
 
      12-09-2006
kwikius wrote:

> Steven T. Hatton wrote:
>
>> What does the Standard specify it should point to, or is that specified?

>
> The only time you can use const_cast on some object reference is when
> you *know* that the real object is not const. Any const cast on a
> physically const object results in undefined behaviour. The better
> option on stuff you *own* is to use mutable:


My inclination is to try and figure out what I'm doing wrong, and fix it.
I've come across a couple places recently where experts "Josuttis,
Vandevoorde, Blanchette, Summerfield, etc." have used const_cast<> and
defended their choice. Nonetheless...

> That is my understanding
> anyway:
>
> struct my{
> private:
> int n;
> public:
> my():n(1),y(2){}
> // mod is declared as a const function
> // but maybe want to keep internal count
> void mod1()const
> {
> // dodgy If someone declares a const my
> my& mm = const_cast<my&>(*this);
> ++mm.n;
> }
> private:
> mutable int y;
> public:
> void mod2()const
> { // always ok
> ++y;
> }
> };
>
> int main()
> {
> my x;
> x.mod1();
> x.mod2();
> }


I know what Stroustrup had to say about mutable. He thinks it is almost
always a bad idea because it defeats the purpose of const. But the same
goes for const_cast only more so.

Here's my hack on the subsequent example from
http://cartan.cas.suffolk.edu/moin/OopDocbookWiki

#include <iostream>
using namespace std;

void el() {
const int L = 99;
int * pL = const_cast<int*>(&L);
*pL = 101;
cout << L << '\t' << &L << endl;
cout << *pL << '\t' << pL << endl;
}

void em() {
const int M = 29;
int * pM = const_cast<int*>(&M);
*pM = 66;
cout << M << '\t' << &M << endl;
cout << *pM << '\t' << pM << endl;
}

const int N = 22;
void en() {

int * pN = const_cast<int*>(&N);
*pN = 33;
cout << N << '\t' << &N << endl;
cout << *pN << '\t' << pN << endl;
}

int main() {
el();
em();
en();
}
/*
Sat Dec 09 09:06:25:> c++ -o cc1 constcast1.cpp
hattons@ljosalfr:~/code/c++/ezust/src/casts/
Sat Dec 09 09:06:46:> ./cc1
99 0xbfc2ccb0
101 0xbfc2ccb0
29 0xbfc2ccb0
66 0xbfc2ccb0
Segmentation fault

*/

The authors are saying that[*] "const int is in stack storage class". I
believe that is old-timer talk meaning 'automatic variable' but I'm not
sure. I have a couple of observations about the code above, and the result
of executing it. The addresses of all the variables which were printed
prior to the segfault are the same. The segfault happened when the
variable was placed at global scope. If nothing else, it shows that
undefined behavior is undefined.
[*]If I understand correctly.
--
NOUN:1. Money or property bequeathed to another by will. 2. Something handed
down from an ancestor or a predecessor or from the past: a legacy of
religious freedom. ETYMOLOGY: MidE legacie, office of a deputy, from OF,
from ML legatia, from L legare, to depute, bequeath. www.bartleby.com/61/
 
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kwikius
Guest
Posts: n/a
 
      12-09-2006

Steven T. Hatton wrote:

> Here's my hack on the subsequent example from
> http://cartan.cas.suffolk.edu/moin/OopDocbookWiki



>
> #include <iostream>
> using namespace std;
>
> void el() {
> const int L = 99;
> int * pL = const_cast<int*>(&L); // use pL and its undefined
> *pL = 101; //undefined behaviour
> cout << L << '\t' << &L << endl;
> cout << *pL << '\t' << pL << endl;
> }
>
> void em() {
> const int M = 29;
> int * pM = const_cast<int*>(&M); // same applies
> *pM = 66; //undefined behaviour
> cout << M << '\t' << &M << endl;
> cout << *pM << '\t' << pM << endl;
> }
>
> const int N = 22;
> void en() {
>
> int * pN = const_cast<int*>(&N); // and here
> *pN = 33; //undefined behaviour
> cout << N << '\t' << &N << endl;
> cout << *pN << '\t' << pN << endl;
> }


All undefined . The only time you can use const_cast is if you know
something is not but looks like it is:


// object pointed by p is 'logically const'
int f( const int & p)
{
int & pp = const_cast<int&>(p);
++pp; // behaviour here dependent on whether
// object referenced by pp is "physically const" or not
// if object is really const... undefined behaviour

return pp;
}

int main()
{
int x=0;
f(x); // OK... x is not really const

const int y =0;
f(y); // Bad.. y Is really const ...undefined behaviour
}

regards
Andy Little

 
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