为什么编译错误“未定义的引用”

Question

我有一个类并按下面的方式使用它，但是当我编译它时，我得到了错误“对'RemoteControl::key‘的未定义的引用，我没有起诉为什么，有人帮助我吗?我认为这个类非常简单，但是我不知道为什么编译是错误。

#include <lnp/lnp.h>
#include <conio.h>  // for the delay() function
#include <remote.h>
#include <sys/program.h>
#include <c++/Motor.H>
#include <c++/Sound.H>

extern int remote_control_handler(unsigned int etype, unsigned int key);
extern int checkMessage(int argc, char **argv);

class RemoteControl {
public:
    static unsigned int key;

    static const void init() {
        lr_set_handler(remote_control_handler);
        execi(&checkMessage, 0, 0, PRIO_NORMAL, DEFAULT_STACK_SIZE);
    }
};

wakeup_t remote_control_pressed_message(wakeup_t data) {
    return lnp_rcx_message >= 1 && lnp_rcx_message <= 3;
}

int checkMessage(int argc, char **argv) {
    while (!shutdown_requested()) {
        clear_msg();
        wait_event(&remote_control_pressed_message, 0);

        if (lnp_rcx_message == 1) {
            RemoteControl::key = LRKEY_M1;
        }
        else if (lnp_rcx_message == 2) {
            RemoteControl::key = LRKEY_M2;
        }
        else if (lnp_rcx_message == 3) {
            RemoteControl::key = LRKEY_M3;
        }
    }
    return 0;
}

 int remote_control_handler(unsigned int etype, unsigned int key) {
     RemoteControl::key = 0;

     if (etype == LREVT_KEYON) {
         switch(key) {
             case LRKEY_STOP:
             case LRKEY_BEEP:
             case LRKEY_A1:
             case LRKEY_A2:
             case LRKEY_B1:
             case LRKEY_B2:
             case LRKEY_C1:
             case LRKEY_C2:
                 RemoteControl::key = key;
                 break;

             default:
                 RemoteControl::key = 0;
                 break;
        }
    }
    return 1;
}

wakeup_t remote_control_pressed_key(wakeup_t data) {
    return RemoteControl::key == data;
}

wakeup_t remote_control_pressed(wakeup_t data) {
    return RemoteControl::key != 0;
}

int main(int argc, char **argv) {
    RemoteControl::init();
    Motor *m = new Motor(Motor::B);

    while (!shutdown_requested()) {
        RemoteControl::key = 0;
        clear_msg();
    wait_event(&remote_control_pressed, 0);

        if (RemoteControl::key == LRKEY_STOP) {
            program_stop(1);
        }
        else if (RemoteControl::key == LRKEY_A1) {
            m->forward();
        }
        else if (RemoteControl::key == LRKEY_M1) {
            Sound::beep();
        }
    }
    return 0;
}

Answer 1

您只声明静态成员变量key，但从不定义它。

它必须在类之外单独定义(以及在源文件中)：

unsigned int RemoteControl::key;