129. Composition

A MotorController class uses PWM and Encoder as member objects:

class MotorController {
   PWM pwm;
   Encoder enc;
public:
   MotorController(int pwm_pin, int enc_pin)
       : pwm(pwm_pin), enc(enc_pin) {}
   void set_speed(int s) { pwm.set(s); }
   int get_pos() { return enc.read(); }
};

What relationship does this demonstrate?