具有静态和const成员变量的对象

Question

In an embedded c project I have several motors of different type. The implementation has static and const variables, and supports sub types.

It looks something like this (simplified):

// motor.h

// to be global/static
typedef struct
{
    int distance;
} motor_instance;

// to be const
typedef const struct motor_hwToken_s
{
    motor_instance *self;
    void (*setSpeedFunction)(const struct motor_hwToken_s *token, int speed);
    int hwPin;
} motor_hwToken_t;

extern motor_hwToken_t motor_A;
void setSpeed(const struct motor_hwToken_s *token, int speed);


// motor.c
void setSpeed(const struct motor_hwToken_s *token, int speed)
{
    token -> setSpeedFunction(token, speed);
}

// specialization for DC-motors
void setSpeed_dcMotor(const struct motor_hwToken_s *token, int speed);

// global variable
motor_dc_instance motor_A_instance;

// const variable, known at compile time
motor_hwToken_t motor_A =
{
    .self = &motor_A_instance,
    .setSpeedFunction = setSpeed_dcMotor,
    .hwPin = 0x04       // define an IO-Pin of some sort
};


// main.c
void main(void)
{
    setSpeed(&motor_A, 20);
}

While the code works, it's quite hard to read. I assume an object oriented language would make more sense?

So my question is:

1. How could I implement this in c++?
2. Will the cpp compiler be able to optimize the const variables (eg put them in flash)?

Answer 1

The following class structure would be suggested. Instead of function pointer just re-implement the set speed function.

//! Motor interface class with abstract functions.
class Motor {
  public:
    virtual ~Motor() = default;
    void set_hwPin(int pin) { hwPin = pin; };
    virtual void set_speed(int speed) = 0;

  private:
    int hwPin;
};

class MotorDC : public Motor {
  public:
    ~MotorDC() override = default;
    void set_speed(int speed) override {
      // Actual implementation for DC motors.
    }
};

class MotorAC : public Motor {
  public:
    ~MotorAC() override = default;
    void set_speed(int speed) override {
      // Actual implementation for AC motors.
    }
};

int main() 
{
  MotorDC mDC;
  mDC.set_pin(1);

  MotorAC mAC;
  mAC.set_pin(2);

  // To demonstrate use a pointer to the abstract base class.
  Motor* m;
  m = &mAC;
  m->set_speed(10);

  m = &mDC;
  m->set_speed(20);
};