undefined reference to `vtable for Base'

[jimjim]

Hello all,

class Base {
public:
virtual void f(); };

class Derived : public Base{
private:
int i; };

int main() {
Derived arrayOfDerived[10]; }

/cygdrive/c/DOCUME~1/ADMINI~1/LOCALS~1/Temp/ccwM5eGM.o(.rdata$_ZTV7Derived[vtabl
e for Derived]+0x8):wrong.cpp: undefined reference to `Base::f()'
/cygdrive/c/DOCUME~1/ADMINI~1/LOCALS~1/Temp/ccwM5eGM.o(.text$_ZN4BaseC2Ev[Base::
Base[not-in-charge]()]+0x8):wrong.cpp: undefined reference to `vtable for
Base'
collect2: ld returned 1 exit status

Why doesnt this piece of code compile?

TIA

 

[Victor Bazarov]

class Base {
public:
virtual void f(); };

The rule is simple: a virtual function shall be defined or declared pure
or both. Yours is not pure. Hence it _must_ be defined.

class Derived : public Base{
private:
int i; };

int main() {
Derived arrayOfDerived[10]; }

/cygdrive/c/DOCUME~1/ADMINI~1/LOCALS~1/Temp/ccwM5eGM.o(.rdata$_ZTV7Derived[vtabl
e for Derived]+0x8):wrong.cpp: undefined reference to `Base::f()'
/cygdrive/c/DOCUME~1/ADMINI~1/LOCALS~1/Temp/ccwM5eGM.o(.text$_ZN4BaseC2Ev[Base::
Base[not-in-charge]()]+0x8):wrong.cpp: undefined reference to `vtable for
Base'
collect2: ld returned 1 exit status

Why doesnt this piece of code compile?

But it does. The program fails to link, not compile.

The compiler assumes that the function is defined somewhere else and
lets the linker resolve it. The linker can't find any definition for
it and complains. What you have here is a simple violation of the
rules of the language, that's all.

V

 

[ravinderthakur]

you have forgotten to define the function f(). plz define it either in
base classs or in the derived class.

 

[jimjim]

The rule is simple: a virtual function shall be defined or declared pure

or both. Yours is not pure. Hence it _must_ be defined.

thx for the reply

int main() {
Derived arrayOfDerived[10]; }

if I make the following change the program links ! Why is this?

int main() {
Derived arrayOfDerived(); }

 

[jimjim]

you have forgotten to define the function f(). plz define it either in
base classs or in the derived class.

thx for the reply

 

[Serge Paccalin]

Le jeudi 11 août 2005 à 13:06, jimjim a écrit dans comp.lang.c++ :

int main() {
Derived arrayOfDerived[10]; }

if I make the following change the program links ! Why is this?

int main() {
Derived arrayOfDerived(); }

Because there is no object instantiated now.
You just declared a parameter-less function returning a Derived.

--
___________ 11/08/2005 14:33:46
_/ _ \_`_`_`_) Serge PACCALIN -- sp ad mailclub.net
\ \_L_) Il faut donc que les hommes commencent
-'(__) par n'être pas fanatiques pour mériter
_/___(_) la tolérance. -- Voltaire, 1763

 

[jimjim]

Because there is no object instantiated now.

You just declared a parameter-less function returning a Derived.

I can only lough with my question

 

[jimjim]

Because there is no object instantiated now.

You just declared a parameter-less function returning a Derived.

didnt really know that I can declare a function in the body of another
function! What is the benefit of this?

TIA

 

[Victor Bazarov]

didnt really know that I can declare a function in the body of another
function! What is the benefit of this?

Limit the scope in which that name appears. You can declare a function
at any level. The deeper the scope nesting the less pollution to the
outside scopes.

V

 

[jimjim]

thx