This Pointer

Inside a member function, this is a pointer to the object the member function was invoked on and cannot be reassigned:

class Circle {
public:
  double area() { return 3.14 * this->radius * this->radius; }
  double radius;
};

Circle c;
c.radius = 2.0;
c.area();  // this == &c

Fields can be accessed without this since the compiler adds it implicitly. this->radius and radius are equivalent:

double area() { return 3.14 * radius * radius; }

this is needed to differentiate a parameter and field with the same name:

class Circle {
public:
  void setRadius(double radius) {
    this->radius = radius;
  }

  double radius;
};

this can be dereferenced to get a reference to the object itself. Returning *this lets member function calls be chained:

class Circle {
public:
  Circle& setRadius(double r) {
    radius = r;
    return *this;
  }

  Circle& scale(double factor) {
    radius *= factor;
    return *this;
  }

  double radius;
};

Circle c;
c.setRadius(2.0).scale(3.0);  // radius = 6