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]

Re: clarification - calling virtual function from destructor

On Fri, 6 Feb 2004 12:16:46 +0200, "lallous" <lallous@lgwm.o rg> wrote:
[color=blue]
>class A
>{
>public:
>virtual void Func1() = 0;
>void Func1Caller()
>{
>Func1();
>}
>~A()
>{
>Func1Caller( );
>}
>};[/color]

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]

Re: clarification - calling virtual function from destructor

"Alf P. Steinbach" <alfps@start.no > wrote in message
news:40236bee.1 00362265@News.C IS.DFN.DE...[color=blue]
> On Fri, 6 Feb 2004 12:16:46 +0200, "lallous" <lallous@lgwm.o rg> wrote:
>[color=green]
> >class A
> >{
> >public:
> >virtual void Func1() = 0;
> >void Func1Caller()
> >{
> >Func1();
> >}
> >~A()
> >{
> >Func1Caller( );
> >}
> >};[/color]
>[/color]
[color=blue]
> The destructor here has undefined behavior because it's calling
> a pure virtual function.[/color]
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.
[color=blue]
>
> With a good C++ compiler this will cause a run-time error, but
> formally it could do anything (including seemingly "working").[/color]
It did produce a runtime error.
[color=blue]
> Hint: get your indentation in order before worrying about such
> things, and also see the FAQ, which answers such questions.[/color]
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]

Re: clarification - calling virtual function from destructor

On Fri, 6 Feb 2004 12:47:13 +0200, "lallous" <lallous@lgwm.o rg> wrote:
[color=blue]
>I read C++ Lite FAQ but didn't know how to solve.[/color]

See the section on constructors.

[lilburne]

Re: clarification - calling virtual function from destructor



lallous wrote:[color=blue]
> "Alf P. Steinbach" <alfps@start.no > wrote in message
> news:40236bee.1 00362265@News.C IS.DFN.DE...
>
>[color=green]
>>The destructor here has undefined behavior because it's calling
>>a pure virtual function.[/color]
>
> 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.
>[/color]

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

[james]

Re: clarification - calling virtual function from destructor

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]

Re: clarification - calling virtual function from destructor


"james" <jj6066@dcs.war wick.ac.uk> wrote in message
news:c0017c$odl $1@mail.dcs.war wick.ac.uk...[color=blue]
> 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;
> }[/color]


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]

Re: clarification - calling virtual function from destructor

On Fri, 06 Feb 2004 12:16:46 +0200, lallous wrote:
[color=blue]
> 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?[/color]

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]

Re: clarification - calling virtual function from destructor

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" <jj6066@dcs.war wick.ac.uk> ¦b¶l¥ó
news:c0017c$odl $1@mail.dcs.war wick.ac.uk ¤¤¼¶¼g...[color=blue]
> Every class is better has own constructor and deconstructor itself, even[/color]
if[color=blue]
> 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;
> }
>
>[/color]

[james]

Re: clarification - calling virtual function from destructor

I talked about virtual function, so it is not serious for formal porgram,
anyway thanks a lot
"Sharad Kala" <no.spam_sharad k_ind@yahoo.com > ¦b¶l¥ó
news:c002gs$10k j6e$1@ID-221354.news.uni-berlin.de ¤¤¼¶¼g...[color=blue]
>
> "james" <jj6066@dcs.war wick.ac.uk> wrote in message
> news:c0017c$odl $1@mail.dcs.war wick.ac.uk...[color=green]
> > Every class is better has own constructor and deconstructor itself, even[/color][/color]
if[color=blue][color=green]
> > compiler allow you do that. Class A func1 alway be called In your code.[/color][/color]
I[color=blue][color=green]
> > 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;
> > }[/color]
>
>
> 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.
>
>[/color]

[james]

Re: clarification - calling virtual function from destructor

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]

Re: clarification - calling virtual function from destructor

lilburne wrote:
[color=blue]
>
> When you enter A's destructor B has already been destroyed, it is no
> more, it is an ex-B.
>[/color]

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