C++ · OOP: inheritance and virtual functions

In Python every method call is resolved at runtime. In C++ you have to ask for that with virtual:

• A virtual method called through a base pointer or reference runs the derived class’s version. Without virtual, the base version runs.
• Mark overrides with override so the compiler catches signature typos.
• = 0 makes a method pure virtual, and the class becomes abstract (you can’t create one directly).
• A base class used through pointers needs a virtual destructor (Core Guideline C.35). Otherwise deleting through a base pointer skips the derived destructor.

#include <iostream>
#include <memory>
#include <vector>

struct Shape {
    virtual ~Shape() = default;
    virtual double area() const = 0;              // pure virtual
    virtual const char* name() const { return "shape"; }
    const char* plainName() const { return "shape (non-virtual)"; }
};

struct Circle : Shape {
    double r;
    explicit Circle(double r) : r(r) {}
    double area() const override { return 3.14159 * r * r; }
    const char* name() const override { return "circle"; }
};

struct Rect : Shape {
    double w, h;
    Rect(double w, double h) : w(w), h(h) {}
    double area() const override { return w * h; }
    const char* name() const override { return "rect"; }
};

int main() {
    std::vector<std::unique_ptr<Shape>> shapes;
    shapes.push_back(std::make_unique<Circle>(1.0));
    shapes.push_back(std::make_unique<Rect>(2.0, 3.0));
    for (const auto& s : shapes)
        std::cout << s->name() << " / " << s->plainName() << " -> " << s->area() << '\n';
}

Output:

circle / shape (non-virtual) -> 3.14159
rect / shape (non-virtual) -> 6