clarification - calling virtual function from destructor

[lallous]

Hello
class A

{

public:

virtual void Func1() = 0;

void Func1Caller()

{

Func1();

}

~A()

{

Func1Caller();

}

};

class B : public A

{

public:

virtual void Func1()

{

printf("this is func1\n");

}

};

int main()

{

B t;

return 0;

}

It seems that VC is not allowing Func1Caller() to call Func1() from the
destructor.
Don't know if this a C++ issue or a compiler issue.

Can someone clarify and provide a workaround?

--
Elias

[Alf P. Steinbach]

On Fri, 6 Feb 2004 12:16:46 +0200, "lallous" <la*****@lgwm.org> wrote:

class A
{
public:
virtual void Func1() = 0;
void Func1Caller()
{
Func1();
}
~A()
{
Func1Caller();
}
};

The destructor here has undefined behavior because it's calling
a pure virtual function.

With a good C++ compiler this will cause a run-time error, but
formally it could do anything (including seemingly "working").

Hint: get your indentation in order before worrying about such
things, and also see the FAQ, which answers such questions.

[lallous]

"Alf P. Steinbach" <al***@start.no> wrote in message
news:40****************@News.CIS.DFN.DE...

On Fri, 6 Feb 2004 12:16:46 +0200, "lallous" <la*****@lgwm.org> wrote:

class A
{
public:
virtual void Func1() = 0;
void Func1Caller()
{
Func1();
}
~A()
{
Func1Caller();
}
};

The destructor here has undefined behavior because it's calling
a pure virtual function. But isn't the pure virtual function defined through class B?

And you cannot create an instance of class A unless its pure virtual method
is defined.

With a good C++ compiler this will cause a run-time error, but
formally it could do anything (including seemingly "working"). It did produce a runtime error.
Hint: get your indentation in order before worrying about such
things, and also see the FAQ, which answers such questions.

The code is idented, but the NG composer got it unidented.

I read C++ Lite FAQ but didn't know how to solve.

--
Elias

[Alf P. Steinbach]

On Fri, 6 Feb 2004 12:47:13 +0200, "lallous" <la*****@lgwm.org> wrote:

I read C++ Lite FAQ but didn't know how to solve.

See the section on constructors.

[lilburne]

lallous wrote:

"Alf P. Steinbach" <al***@start.no> wrote in message
news:40****************@News.CIS.DFN.DE...

The destructor here has undefined behavior because it's calling
a pure virtual function.

But isn't the pure virtual function defined through class B?

And you cannot create an instance of class A unless its pure virtual method
is defined.

When you enter A's destructor B has already been destroyed, it is no
more, it is an ex-B.

[james]

Every class is better has own constructor and deconstructor itself, even if
compiler allow you do that. Class A func1 alway be called In your code. I
revised your code for you. every class will call func1 itself. then you
don't need to name"virturl" in class B.
class A
{
public:
virtual void Func1()
{
printf("this is class A");
};
void Func1Caller()
{
Func1();
}
~A()
{
Func1Caller();
}
};

class B : public A
{
public:
B();
~B();
void Func1(){ printf("this is class B\n");}
B::B() {}
B::~B(){ Func1Caller();}
};

int main()

{
B t;
return 0;
}

[Sharad Kala]

"james" <jj****@dcs.warwick.ac.uk> wrote in message
news:c0**********@mail.dcs.warwick.ac.uk...

Every class is better has own constructor and deconstructor itself, even if
compiler allow you do that. Class A func1 alway be called In your code. I
revised your code for you. every class will call func1 itself. then you
don't need to name"virturl" in class B.
class A
{
public:
virtual void Func1()
{
printf("this is class A");
};
void Func1Caller()
{
Func1();
}
~A()
{
Func1Caller();
}
};

class B : public A
{
public:
B();
~B();
void Func1(){ printf("this is class B\n");}
B::B() {}
B::~B(){ Func1Caller();}
};

int main()

{
B t;
return 0;
}

The code you posted won't even compile dear friend!
As pointed by others calling pure virtual function inside a destructor is
calling in for undefined behavior.

[Martijn Lievaart]

On Fri, 06 Feb 2004 12:16:46 +0200, lallous wrote:

It seems that VC is not allowing Func1Caller() to call Func1() from the
destructor.
Don't know if this a C++ issue or a compiler issue.

Can someone clarify and provide a workaround?

It's normal C++ behaviour. At the time of the destructor (the same goes
for the constrctor) the dynamic type is set to that of the destructor.
What does that mean?

class A
{
public:
virtual f1() { puts("A"); }
};

class B
{
public:
virtual f1() { puts("B"); }
~B()
{
f1(); // Prints B, we are a B, we are no longer a C
A *p = this;
p->f1(); // Again prints B, virtual functions still work,
// we are a B, so calls B::f1
}
};

class C
{
public:
virtual f1() { puts("C"); }
};

int main();
{
C c;
}

When C gets destroyed, first the destuctor for C is executed, then the
destructor for the B subobject *but* with it's type set to B, then the A
subobject is destroyed, but with it's type set to A.

Think about it. The C object is already destroyed, it would make a fine
mess if virtual functions worked the way you assumed they do.

HTH,
M4

[james]

another example is more clear about virtual function
declareation is as yours
B mean;
A meam1;
B *meam2;
meam2=&meam1;
meam2->Func1(); // callB::Func1();
meam2=&A;
meam2->Func1(); //call A::Func1();
"james" <jj****@dcs.warwick.ac.uk> ¦b¶l¥ó
news:c0**********@mail.dcs.warwick.ac.uk ¤¤¼¶¼g...

Every class is better has own constructor and deconstructor itself, even if compiler allow you do that. Class A func1 alway be called In your code. I
revised your code for you. every class will call func1 itself. then you
don't need to name"virturl" in class B.
class A
{
public:
virtual void Func1()
{
printf("this is class A");
};
void Func1Caller()
{
Func1();
}
~A()
{
Func1Caller();
}
};

class B : public A
{
public:
B();
~B();
void Func1(){ printf("this is class B\n");}
B::B() {}
B::~B(){ Func1Caller();}
};

int main()

{
B t;
return 0;
}

[james]

I talked about virtual function, so it is not serious for formal porgram,
anyway thanks a lot
"Sharad Kala" <no*****************@yahoo.com> ¦b¶l¥ó
news:c0*************@ID-221354.news.uni-berlin.de ¤¤¼¶¼g...

"james" <jj****@dcs.warwick.ac.uk> wrote in message
news:c0**********@mail.dcs.warwick.ac.uk...

Every class is better has own constructor and deconstructor itself, even if compiler allow you do that. Class A func1 alway be called In your code. I revised your code for you. every class will call func1 itself. then you
don't need to name"virturl" in class B.
class A
{
public:
virtual void Func1()
{
printf("this is class A");
};
void Func1Caller()
{
Func1();
}
~A()
{
Func1Caller();
}
};

class B : public A
{
public:
B();
~B();
void Func1(){ printf("this is class B\n");}
B::B() {}
B::~B(){ Func1Caller();}
};

int main()

{
B t;
return 0;
}

The code you posted won't even compile dear friend!
As pointed by others calling pure virtual function inside a destructor is
calling in for undefined behavior.

[james]

then I agree with Sharard Kala' opion. It is a bed idea to put virtual
function in destructor. see my another example at previou post.

[red floyd]

lilburne wrote:

When you enter A's destructor B has already been destroyed, it is no
more, it is an ex-B.

It has ceased to "B"? Of course, it could just be pining for the fjords.