Undefined symbols: vtable for Foo

[r]

I compile this on a Max OSX with
GCC version 1151, based on gcc version 3.1 20020420

g++ foo.cpp
ld: Undefined symbols: <-- ERROR
vtable for Foo

Any idea why?
-r
**************** contents of foo.cpp **********

class Foo {

protected:
void setit(int);

public:
virtual void bar(); // only a subclass implements
};
class Bird : public Foo {

public:
void bar();
};

// ################

int a;

void Foo::setit(int i) {
a = i;
}

void Bird::bar() {
Foo::setit(8);
}

int main(int argc, char **argv) {
Bird *b = new Bird();
b->bar();
}

// end

[Gianni Mariani]

r wrote:

I compile this on a Max OSX with
GCC version 1151, based on gcc version 3.1 20020420

g++ foo.cpp
ld: Undefined symbols: <-- ERROR
vtable for Foo

Any idea why?
-r
**************** contents of foo.cpp **********

class Foo {

protected:
void setit(int);

public:
virtual void bar(); // only a subclass implements
try:
virtual void bar() = 0;
virtual void bar() indicates that the compiler expects there to be an
implementation.

};
class Bird : public Foo {

public:
void bar();
};

// ################

int a;

void Foo::setit(int i) {
a = i;
}

void Bird::bar() {
Foo::setit(8);
}

int main(int argc, char **argv) {
Bird *b = new Bird();
b->bar();
}

// end

[Rob Williscroft]

r wrote in news:38*************************@posting.google.co m:

I compile this on a Max OSX with

GCC version 1151, based on gcc version 3.1 20020420

g++ foo.cpp
ld: Undefined symbols: <-- ERROR
vtable for Foo

[snip]
class Foo {

protected:
void setit(int);

public:
change this:
virtual void bar(); // only a subclass implements
To:

virtual void bar() = 0;

};
class Bird : public Foo {

public:
void bar();
};

// ################
int a;

[snip]

Adding the = 0; to the declaration of bar() above makes bar()
a pure virtual and Foo an abstract class. In effect telling
the compiler you want to derive from Foo but NOT create any
instances of Foo. The compiler can then omit creating a vtable
for Foo and your linker error will go away.

Alternatively provide an defenition for Foo::bar()

voif Foo::bar()
{
}

Do it this way if you want to create instances of Foo.

HTH

Rob.
--
http://www.victim-prime.dsl.pipex.com/