#include <iostream>
using namespace std;
class Person{
public:
virtual void print () { cout << "In parent" << endl; }
void onlyInPerson() { cout << "Only in Person/parent" <<
endl; }
};
class Student: public Person{
public:
Student() { cout << "Student constructor." << endl; }
void print() { cout << "In child" << endl; }
void onlyInChild() { cout << "Only in Child/Student" <<
endl; }
};
int main()
{
Person* p ;
Student* s;
s->onlyInChild( );
return 0;
}
Dec 14 '07
17  1489  am************* ********@gmail. com wrote:
Isin't the garbage this pointer dereferenced to to call the member
function onlyInChild() ?
Maybe, maybe not. Who cares?
There. Is. No. Defined. Behavior. For. Undefined. Behavior.
Say that over and over to yourself until you GET IT.
Brian
On Dec 14, 5:35 pm, "Default User" <defaultuse...@ yahoo.comwrote:
amit.bolakani.t echni...@gmail. com wrote:
Isin't the garbage this pointer dereferenced to to call the member
function onlyInChild() ?
Maybe, maybe not. Who cares?
There. Is. No. Defined. Behavior. For. Undefined. Behavior.
Say that over and over to yourself until you GET IT.
Brian
No worries Brian :) GOT IT!
Thanks for all your help guys. Appreciate it.
On 2007-12-14 23:33, am************* ********@gmail. com wrote:
On Dec 14, 5:05 pm, Vaclav Haisman <v.hais...@sh.c vut.czwrote:
>amit.bolakani. techni...@gmail .com wrote, On 14.12.2007 22:29:
#include <iostream>
using namespace std;
class Person{
public:
virtual void print () { cout << "In parent" << endl; }
void onlyInPerson() { cout << "Only in Person/parent" <<
endl; }
};
class Student: public Person{
public:
Student() { cout << "Student constructor." << endl; }
void print() { cout << "In child" << endl; }
void onlyInChild() { cout << "Only in Child/Student" <<
endl; }
};
int main()
{
Person* p ;
Student* s;
s->onlyInChild( );
return 0;
}
The programme compiles because it is well-formed and probably even runs
because the methods actually do not touch anything in the object itself but
your programme is invoking undefined behaviour.
--
VH
Ok, so I understand that its undefined behavior. However, is the
garbage this pointer dereferenced to call the member method?
Do you see 'this' (or any other members) mentioned anywhere in
onlyInChild()? If you do not, why would it be dereferenced?
--
Erik Wikström
On 14 Dec, 22:03, amit.bolakani.t echni...@gmail. com wrote:
On Dec 14, 4:59 pm, tragomaskhalos <dave.du.verg.. .@logicacmg.com >
wrote:
On 14 Dec, 21:45, amit.bolakani.t echni...@gmail. com wrote:
On Dec 14, 4:34 pm, David Côme <davidc...@wana doo.frwrote:
On Fri, 14 Dec 2007 22:29:32 +0100, <amit.bolakani. techni...@gmail .com>
wrote:
#include <iostream>
using namespace std;
class Person{
public:
virtual void print () { cout << "In parent" << endl; }
void onlyInPerson() { cout << "Only in Person/parent" <<
endl; }
};
class Student: public Person{
public:
Student() { cout << "Student constructor." << endl; }
void print() { cout << "In child" << endl; }
void onlyInChild() { cout << "Only in Child/Student" <<
endl; }
};
int main()
{
Person* p ;
Student* s;
s->onlyInChild( );
return 0;
}
I'm not sure but for me it's an undefined behaviour
It seems to compile and run fine! I'm pretty surprised and it throws
me off a bit in terms of how it runs fine?- Hide quoted text -
That it happens to work is no surprise - onlyInChild does not access
any members, and the method is not virtual, hence the garbage this
pointer is
not dereferenced. Neverthless it's still undefined behaviour.
Isin't the garbage this pointer dereferenced to to call the member
function onlyInChild() ?- Hide quoted text -
OK, my original reply wasn't detailed enough.
Others have said that this is UB but you are quite correctly
still wondering why it happend to work with your compiler.
The key is that the compiler is likely producing code something akin
to this:
//SOURCE:
// void Student::onlyIn Child() { cout << "Only in Child/Student" <<
endl; }
//COMPILER:
void __Student_onlyI nChild(__this) {
/* ... stuff that NEVER USES __this */
}
//SOURCE s->onlyInChild( ); // nb s is uninitialised ...
//COMPILER:
__Student_onlyI nChild(s); // nb s is NOT dereferenced
HTH
Hi am************* ********@gmail. com wrote:
Ok, so I understand that its undefined behavior. However, is the
garbage this pointer dereferenced to call the member method?
That's just one of the things that is undefined.
Markus
On 2007-12-15 04:36:30 -0500, tragomaskhalos
<da************ *@logicacmg.com said:
>
OK, my original reply wasn't detailed enough.
Others have said that this is UB but you are quite correctly
still wondering why it happend to work with your compiler.
The key is that the compiler is likely producing code something akin
to this:
Maybe. Speculating about why something whose behavior is undefined
happens to do what a naive programmer guessed it would do is generally
pointless. Undefined means undefined. Nothing more. The reason that it
"works" is pure accident. (That's the screed for beginning and
intermediate programmers.)
--
Pete
Roundhouse Consulting, Ltd. ( www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
( www.petebecker.com/tr1book)
On 15 Dec, 13:57, Pete Becker <p...@versatile coding.comwrote :
On 2007-12-15 04:36:30 -0500, tragomaskhalos
<dave.du.verg.. .@logicacmg.com said:
OK, my original reply wasn't detailed enough.
Others have said that this is UB but you are quite correctly
still wondering why it happend to work with your compiler.
The key is that the compiler is likely producing code something akin
to this:
Maybe. Speculating about why something whose behavior is undefined
happens to do what a naive programmer guessed it would do is generally
pointless. Undefined means undefined. Nothing more. The reason that it
"works" is pure accident. (That's the screed for beginning and
intermediate programmers.)
Well that's probably a good philosophy in the general case, but here
the OP accepted the undefinedness of the code but was clearly baffled
as to how it *could ever* have worked; seeing how a compiler might
realistically have treated the code so as to produce the observed
effect
is often valuable - when taken with the usual caveats. Otherwise one
is
left with a helpless feeling that it's all magic, which can only
dishearten the questioner and deter experimentation .
On 2007-12-15 18:14:15 -0500, tragomaskhalos
<da************ *@logicacmg.com said:
On 15 Dec, 13:57, Pete Becker <p...@versatile coding.comwrote :
>On 2007-12-15 04:36:30 -0500, tragomaskhalos
<dave.du.verg. ..@logicacmg.co msaid:
>>OK, my original reply wasn't detailed enough.
Others have said that this is UB but you are quite correctly
still wondering why it happend to work with your compiler.
The key is that the compiler is likely producing code something akin
to this:
Maybe. Speculating about why something whose behavior is undefined
happens to do what a naive programmer guessed it would do is generally
pointless. Undefined means undefined. Nothing more. The reason that it
"works" is pure accident. (That's the screed for beginning and
intermediate programmers.)
Well that's probably a good philosophy in the general case, but here
the OP accepted the undefinedness of the code but was clearly baffled
as to how it *could ever* have worked; seeing how a compiler might
realistically have treated the code so as to produce the observed
effect
is often valuable - when taken with the usual caveats. Otherwise one
is
left with a helpless feeling that it's all magic, which can only
dishearten the questioner and deter experimentation .
Once you're in the realm of undefined behavior, it is all magic, unless
you're skills are advanced enough to analyze machine instructions.
That's not a useful exercise for beginners.
--
Pete
Roundhouse Consulting, Ltd. ( www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
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