Virtual function definition in base class gives error - c++

Question

I had written a set of classes to check composition pattern.
Here is my code:
#include <iostream>
#include <string>
#include <list>
#include "InvalidCompositeException.h"
using namespace std;
class Composite;
class Component {
public:
Component() {}
virtual ~Component() {}
virtual string getName() = 0;
virtual int getNetPrice() = 0;
virtual Composite* getComposite() {
try {
throw myEx;
} catch (InvalidCompositeException& e) {
cout<<"Exception: "<<e.what();
}
return 0;
}
**virtual void add(Component* c);
virtual void remove(Component* c);**
private:
};
class Composite : public Component {
public:
Composite():mChildList(new list<Component*>()) {}
virtual ~Composite() {
mChildList->clear();
delete mChildList;
}
virtual string getName() {return "Composite";}
virtual int getNetPrice() {
list<Component*>::iterator i;
int sum = 0;
for(i=mChildList->begin(); i!= mChildList->end(); ++i) {
sum = sum + (*i)->getNetPrice();
}
return sum;
}
virtual void add(Component* c) {
mChildList->push_back(c);
}
virtual void remove(Component* c) {
mChildList->remove(c);
}
private:
list<Component*>* mChildList;
};
class Container: public Composite {
public:
Container() {}
virtual ~Container() {}
string getName() {
cout<<"container"<<endl;
return "container";
}
};
class Line: public Component {
public:
Line(): mNetPrice(50) {}
~Line() {};
int getNetPrice() { return mNetPrice; }
string getName() {
cout<<"line"<<endl;
return "line";
}
private:
int mNetPrice;
};
class Text: public Component {
public:
Text(): mNetPrice(100) {}
~Text() {};
int getNetPrice() { return mNetPrice; }
string getName() {
cout<<"Text"<<endl;
return "Text";
}
private:
int mNetPrice;
};
int main(void) {
Container* c = new Container();
Line* l = new Line();
Text* t = new Text();
c->add(l);
c->add(l);
c->add(t);
cout<<"total price for c is "<<c->getNetPrice();
l->getComposite();
delete t;
delete l;
delete c;
return EXIT_SUCCESS;
}
My code runs fine except when I add those bold lines in my parent class that i receive errors
undefined reference to `vtable for Component' // on this line virtual ~Component() {}
undefined reference to `Component::add(Component*)'
undefined reference to `Component::remove(Component*)'
I have not define virtual functions to be pure. Then why do I receive those errors even if i don't define them in Line and Text Classes. If I dont add those bold declarations my code works fine. And secondly why error on destructor?

Answer 1

Non-pure virtual functions need to have a definition, even if they are never called (so that the linker has something to put in the vtable). Simply add =0 to make your class abstract, or provide empty definitions.
The error with the destructor is a little more involved, but basically the compiler needs to decide in which object file to place the vtable for your polymophic class -- it usually does this wherever the first non-pure, non-inline virtual function is defined (with more complicated rules where there are no such functions). In this case, you're declaring two out-of-line virtual functions, but never defining them, so the compiler never writes the vtable into an object file.

Answer 2

If there's no implementation in the base class, you need to make them abstract with the =0 in the virtual function declaration. Otherwise, the virtual function table for the base class will try to find bodies - without the =0, it figures they must exist, and that will end up pointing at nothing, causing the linker error.
The destructor error is the same thing - it needs that complete table to find the virtual dtor and the table isn't complete.

Answer 3

Consider:
class foo
{
public:
void doit();
};
foo f;
f.doit(); // linker error, doit isn't defined!
What do you think happens if void doit() becomes virtual doit()? Nothing, same error as you have now. However virtual void doit() = 0 by making it pure will resolve the error. A virtual function is like any other, it must have an implementation.