linker error in virtual function

[Rahul]

Hi Everyone,

I have the following polymorphic classes,

class Shape
{
public : virtual void draw()
{
}
virtual void sample();
};

class Circle : public Shape
{
public : virtual void draw()
{
cout<<"Circle:: draw"<<endl;
}
};

int main()
{
Shape *ptr = new Circle;
ptr->draw();
return(0);
}

and i get a linker error saying undefined external sumbol __sample...
But i haven't really invoked the function sample(). Note that the code
works fine when i change sample() to a non-virtual function prototype,
why is this the case?

Thanks in advance ! ! !

[Pete Becker]

On 2008-04-14 04:06:38 -0400, Michael DOUBEZ <mi************ @free.frsaid:

Rahul a écrit :

>>
and i get a linker error saying undefined external sumbol __sample...
But i haven't really invoked the function sample(). Note that the code
works fine when i change sample() to a non-virtual function prototype,
why is this the case?

If you make it virtual, 10.3-8 of the standard requires that:
"A virtual function declared in a class shall be defined or declared
pure or both; but no diagnostic is required."

Here you don't have the diagnostic of missing definition but the linker
requires it (for its virtual table).

Sure you do: the error message from the linker is a diagnostic. And
that's really the only point where this error could be detected.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

[James Kanze]

On Apr 14, 10:06 am, Michael DOUBEZ <michael.dou... @free.frwrote:

Rahul a écrit :

I have the following polymorphic classes,

class Shape
{
public : virtual void draw()
{
}
virtual void sample();
};

class Circle : public Shape
{
public : virtual void draw()
{
cout<<"Circle:: draw"<<endl;
}
};

int main()
{
Shape *ptr = new Circle;
ptr->draw();
return(0);
}

and i get a linker error saying undefined external sumbol __sample...
But i haven't really invoked the function sample(). Note that the code
works fine when i change sample() to a non-virtual function prototype,
why is this the case?

If you make it virtual, 10.3-8 of the standard requires that:
"A virtual function declared in a class shall be defined or
declared pure or both; but no diagnostic is required."

That's just a note; the normative text is in §3.2: "A virtual
function is used if it is not pure." If a function is used,
then there must be a definition (and only one, unless it is a
template or inline). Otherwise, undefined behavior occurs.

I wonder, however, if this shouldn't read: "a virtual function
is used if it is not pure, and the class is instantiated"? (I
can imagine scenarios where you pull in some header which
defines a class with virtual functions, but you don't actually
use the class, nor link against the library which contains the
definitions. As it now reads, this is undefined behavior.)

Here you don't have the diagnostic of missing definition but
the linker requires it (for its virtual table).

That's still a diagnostic. Not required, but in practice, I
think you'll almost always get it.

--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Pete Becker]

On 2008-04-15 05:19:26 -0400, Michael DOUBEZ <mi************ @free.frsaid:

Pete Becker a écrit :

>On 2008-04-14 04:06:38 -0400, Michael DOUBEZ <mi************ @free.frsaid:

>>Rahul a écrit :

and i get a linker error saying undefined external sumbol __sample...
But i haven't really invoked the function sample(). Note that the code
works fine when i change sample() to a non-virtual function prototype,
why is this the case?

If you make it virtual, 10.3-8 of the standard requires that:
"A virtual function declared in a class shall be defined or declared
pure or both; but no diagnostic is required."

Here you don't have the diagnostic of missing definition but the linker
requires it (for its virtual table).

Sure you do: the error message from the linker is a diagnostic. And
that's really the only point where this error could be detected.

Yes, only the linker could locate the missing symbol but I would prefer
a message like:
Error: undefined reference to 'void Shape::sample() '.

That's a tricky one, though: it requires the linker to know how the
compiler mangles names, something that isn't otherwise necessary. On
many systems, the linker deals with multiple compilers for multiple
languages, and has developed over the course of decades, and is too
fragile to make any but the most essential changes.

>
Well, still better than:
foo.o(.eh_frame +0x11): undefined reference to `__gxx_personal ity_v0'
collect2: ld returned 1 exit status

Which is even more cryptic.

What's cryptic about that? <gIt means that, for whatever reason, you
didn't get the exception handling code. Probably by using "gcc" instead
of "g++" on the command line.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)