Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

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

class Shape {
public:
    virtual ~Shape() = default;

// Pure virtual — every concrete shape must implement this
    virtual double area()        const = 0;
    virtual double perimeter()   const = 0;
    virtual std::string describe() const = 0;

// Non-virtual template method uses virtual helpers
    void report() const {
        std::cout << describe()
                  << "  area="      << area()
                  << "  perimeter=" << perimeter() << "
";
    }
};

class Circle final : public Shape {
public:
    explicit Circle(double r) : r_(r) {}
    double area()      const override { return M_PI * r_ * r_; }
    double perimeter() const override { return 2 * M_PI * r_; }
    std::string describe() const override { return "Circle(r=" + std::to_string(r_) + ")"; }
private:
    double r_;
};

class Rectangle : public Shape {
public:
    Rectangle(double w, double h) : w_(w), h_(h) {}
    double area()      const override { return w_ * h_; }
    double perimeter() const override { return 2 * (w_ + h_); }
    std::string describe() const override {
        return "Rect(" + std::to_string(w_) + "x" + std::to_string(h_) + ")";
    }
private:
    double w_, h_;
};

int main() {
    std::vector<std::unique_ptr<Shape>> shapes;
    shapes.push_back(std::make_unique<Circle>(5.0));
    shapes.push_back(std::make_unique<Rectangle>(4.0, 6.0));

for (const auto& s : shapes)
        s->report();   // virtual dispatch selects correct overrides

return 0;
}