Demo on using `const`.

const_demo.cpp

#include <iostream>

static int outside_variable{0};

class Demo {
    int val;
    public:
    static const int inside{1};
        Demo(int v = 0) {
            val = v;
        }
        
        void f1() {
            std::cout << "f1()\n";
            std::cout << "static int outside_variable: " << outside_variable << '\n';
        }
        
        void f2() const {
            std::cout << "f2()\n";
            int i = 0;
            std::cout << "Current i: " << i << '\t';
            ++i;
            std::cout << "Current i: " << i << '\t';
            // ++outside_variable; // doesn't work
            // Does work with const_cast
            std::cout << "Current outside_variable: " << outside_variable << '\n';
            ++outside_variable;
            // int &modifiable = const_cast <int &> (outside_variable);
            // modifiable = 20;
            std::cout << "Current outside_variable: " << outside_variable << '\n';
        }

        void f3() {
            std::cout << "f3()\n";
            std::cout << "static int inside: " << inside << '\n';
            int& writable = const_cast<int&>(inside);
            writable = 20;
            std::cout << "static int inside: " << inside << '\n';
        }
};

int main() {
    const Demo d;
    Demo c;
    c.f1();
    d.f2();
    c.f1();
    c.f3();
    return 0;
}