C++ Undefined Reference to Vtable and Inheritance

C++ Undefined Reference to vtable and inheritance

Why the error & how to resolve it?

You need to provide definitions for all virtual functions in class A. Only pure virtual functions are allowed to have no definitions.

i.e: In class A both the methods:

virtual ~A();
virtual void doWork();

should be defined(should have a body)

e.g.:

A.cpp

void A::doWork()
{
}
A::~A()
{
}

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Caveat:
If you want your class A to act as an interface(a.k.a Abstract class in C++) then you should make the method pure virtual.

virtual void doWork() = 0;

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Good Read:

What does it mean that the "virtual table" is an unresolved external?

When building C++, the linker says my constructors, destructors or virtual tables are undefined.

Undefined reference to vtable

So, I've figured out the issue and it was a combination of bad logic and not being totally familiar with the automake/autotools world. I was adding the correct files to my Makefile.am template, but I wasn't sure which step in our build process actually created the makefile itself. So, I was compiling with an old makefile that had no idea about my new files whatsoever.

Thanks for the responses and the link to the GCC FAQ. I will be sure to read that to avoid this problem occurring for a real reason.

Undefined Reference to vTable for constructor and destructor

Two things:

1) This code is very unlikely to be what you want:

 class MinHeap : virtual public AbstractHeap

virtual derivation is only of use when you are doing multiple inheritance (which you aren't here, from what you've shown), and even then it's best avoided.

You simply need:

class MinHeap : public AbstractHeap

I suspect this causes the error.

2) If you have a base class, its destructor should be declared as virtual, so:

virtual ~AbstractHeap();

If not, then when you delete a derived class object, the derived class destructor may be skipped. So you need a virtual destructor in the base class even if it does nothing and has an empty body, because the benefit is in the derived class.

c++ inheritance issues: undefined reference to 'vtable'

As pointed out in the comments, by calling g++ main.cpp you are only compiling main.cpp.

You need to compile all files, then link them together. If you do so, you will see that there are compilation issues in your other cpp files as also pointed out in the comments (virtual and override only belong in the header).

So you need to call the following to compile all files:

g++ main.cpp Base.cpp Derived.cpp -o myapp


undefined reference to vtable with inheritance

You do not define the virtual string ManagedObjects::get() const method anywhere. Either define it as a pure virtual function virtual string get() const = 0; or provide a declaration for it.

Undefined reference error while using inheritance and pure virtual functions

Both inheritance and the presence of pure virtual functions are not the problem. You'd get similar error for

class rectangle {
public:
    rectangle();
    void display();
    float area(){
        int A = 42;
        return A;
    }
};

int main(){

    rectangle rec;
    rec.display();

}

You declare a constructor and you declare a method display and the compiler takes that as a promise that you will provide a definition of them somewhere. The linker cannot find them, hence the error. You have to define those methods.

However, in your code actually both methods are not needed. You can use the compiler generated constructor and you can reuse shape::display, hence you can change rectangle to:

class rectangle : public shape{
public:
    float area(){
        int A;
        A = width*height;
        return A;
    }
};

undefined reference to `vtable for ' namespace inheritance undefined reference to `typeinfo for

The problem is, that you have to define every function you declare. That's why the compiler is complaining, you are not defining anything for draw, move or scale.

If you want Figure to be a complete abstract class (which I think this is what you are trying to do), you can set those function to 0, so you don't have to define them, but the derived classes have to implement them.

//Now you don't need to implement them
virtual void draw() = 0;
virtual void move() = 0;
virtual void scale(double) = 0;

undefined reference to vtable for inheriting classes

As explained at https://gcc.gnu.org/wiki/VerboseDiagnostics#missing_vtable the vtable will be in the same object file as the first non-inline virtual function, which is Layer::start. You haven't defined that function, so the compiler never generated the vtable.

To fix the linker error be sure you have provided a definition for the first non-inline virtual function declared in the class.

Undefined reference to vtable

You need to either make virtual void move(); a pure virtual function:

virtual void move() = 0;

or define Actor::move() for a base class

void Actor::move() 
{
    // do something
}

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