Error "Undefined reference to vtable" in C++: causes and solution

Hello! I'm writing a small project in C++ using polymorphism. I have an abstract base class `Shape` and several derived classes: `Circle`, `Rectangle`. In the base class, a virtual function `area()` is declared. When I try to compile the project with g++, the compiler gives an error: ``` undefined reference to 'vtable for Shape' ``` Here's the simplified code: shape.h: ```cpp #ifndef SHAPE_H #define SHAPE_H class Shape { public: virtual double area() = 0; // pure virtual function virtual ~Shape() {} }; #endif ``` circle.h: ```cpp #include "shape.h" class Circle : public Shape { public: Circle(double r); double area() override; private: double radius; }; ``` circle.cpp: ```cpp #include "circle.h" Circle::Circ