ardupilot/libraries/AP_HAL_Empty/PrivateMember.h

#pragma once

/* Just a stub as an example of how to implement a private member of an
 * AP_HAL module */

#include "AP_HAL_Empty.h"

class Empty::PrivateMember {
public:
    PrivateMember(uint16_t foo);
    void init();
private:
    uint16_t _foo;
};