c++ - 错误 : invalid use of non-static member function ‘int test::hotplug_callback(libusb_context*, libusb_device*, libusb_hotplug_event, void*)’

Question

我从

改编了 libusb 热插拔示例

* libusb example program for hotplug API
* Copyright © 2012-2013 Nathan Hjelm <hjelmn@mac.com>

(下面的代码)作为测试并将其放在一个类中，因为我的真实程序也有同样的问题。我知道如果我将 static 放在它起作用的两个回调前面，但我不希望它们是静态的。我想从回调中访问我的实例变量。这个想法是用户插入一个 usb 设备，我们称它为 usbXYZ。从回调中，我实例化类 usbXYZ 并放入一个 std::map - 用户将其删除，然后我将其从 map 中删除。包含 std::map 及其中的对象的类有一个“更高级别”的方法来写入设备。

请问如何使回调函数工作非静态？如果可能的话，请解释一下，因为我不明白。谢谢。

#include <stdlib.h>
#include <stdio.h>
#include <thread>

#include "libusb-1.0/libusb.h"

class test{
public:
    test() {
        okGo();
    }

private:
    int done = 0;
    libusb_device_handle *handle = NULL;

    int LIBUSB_CALL hotplug_callback(libusb_context *ctx, libusb_device *dev, libusb_hotplug_event event, void *user_data)
    {
        struct libusb_device_descriptor desc;
        int rc;

        (void)ctx;
        (void)dev;
        (void)event;
        (void)user_data;

        rc = libusb_get_device_descriptor(dev, &desc);
        if (LIBUSB_SUCCESS != rc) {
            fprintf (stderr, "Error getting device descriptor\n");
        }

        printf ("Device attached: %04x:%04x\n", desc.idVendor, desc.idProduct);

        if (handle) {
            libusb_close (handle);
            handle = NULL;
        }

        rc = libusb_open (dev, &handle);
        if (LIBUSB_SUCCESS != rc) {
            fprintf (stderr, "Error opening device\n");
        }

        done++;

        return 0;
    }

    int LIBUSB_CALL hotplug_callback_detach(libusb_context *ctx, libusb_device *dev, libusb_hotplug_event event, void *user_data)
    {
        (void)ctx;
        (void)dev;
        (void)event;
        (void)user_data;

        printf ("Device detached\n");

        if (handle) {
            libusb_close (handle);
            handle = NULL;
        }

        done++;

        return 0;
    }

    int okGo(){
        libusb_hotplug_callback_handle hp[2];
        int product_id, vendor_id, class_id;
        int rc;

        vendor_id  = LIBUSB_HOTPLUG_MATCH_ANY;
        product_id = LIBUSB_HOTPLUG_MATCH_ANY;
        class_id   = LIBUSB_HOTPLUG_MATCH_ANY;

        rc = libusb_init (NULL);
        if (rc < 0)
        {
            printf("failed to initialise libusb: %s\n", libusb_error_name(rc));
            return EXIT_FAILURE;
        }

        if (!libusb_has_capability (LIBUSB_CAP_HAS_HOTPLUG)) {
            printf ("Hotplug capabilites are not supported on this platform\n");
            libusb_exit (NULL);
            return EXIT_FAILURE;
        }

        rc = libusb_hotplug_register_callback (NULL, LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED, LIBUSB_HOTPLUG_ENUMERATE, vendor_id,
                                               product_id, class_id, hotplug_callback, NULL, &hp[0]);
        if (LIBUSB_SUCCESS != rc) {
            fprintf (stderr, "Error registering callback 0\n");
            libusb_exit (NULL);
            return EXIT_FAILURE;
        }

        rc = libusb_hotplug_register_callback (NULL, LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT, LIBUSB_HOTPLUG_ENUMERATE, vendor_id,
                                               product_id,class_id, hotplug_callback_detach, NULL, &hp[1]);
        if (LIBUSB_SUCCESS != rc) {
            fprintf (stderr, "Error registering callback 1\n");
            libusb_exit (NULL);
            return EXIT_FAILURE;
        }

        while (done < 20) {
            //rc = libusb_handle_events (NULL);
            if (libusb_handle_events_completed(nullptr, nullptr) != LIBUSB_SUCCESS)
                printf("libusb_handle_events() failed: %s\n", libusb_error_name(rc));
        }

        if (handle) {
            libusb_close (handle);
        }

        libusb_exit (NULL);

        return EXIT_SUCCESS;
    }
};


int main(int argc, char *argv[])
{
    std::unique_ptr<test> testClass1 = std::make_unique<test>();

    //just test
    for (;;)
    {
        //main service loop
        std::this_thread::sleep_for(std::chrono::microseconds(2000000));
    }
}

Answer 1

libusb 是一个C 库，它只能接受非成员函数指针。非成员函数(或 static 成员函数)指针与指向非静态成员函数的指针之间的区别在于，非静态成员函数需要一个对象 被调用，而 C 对 C++“对象”一无所知。

但是使用 libusb 可以解决它，使用 static 成员函数和用户数据指针(您现在传递一个空指针)。

例如

class test
{
    // ...

    static int LIBUSB_CALL static_hotplug_callback_detach(libusb_context *ctx, libusb_device *dev, libusb_hotplug_event event, void *user_data)
    {
        return reinterpret_cast<test*>(user_data)->hotplug_callback_detach(ctx, dev, event);
    }

    int hotplug_callback_detach(libusb_context *ctx, libusb_device *dev, libusb_hotplug_event event)
    {
        // Current code
    }

    // ...

    int okGo()
    {
        // ...

        rc = libusb_hotplug_register_callback (NULL, LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT, LIBUSB_HOTPLUG_ENUMERATE, vendor_id,
                                           product_id,class_id, static_hotplug_callback_detach, this, &hp[1]);

        // ...
    }

请注意，在注册回调时使用了 static 成员函数，并且 this 作为用户数据指针传递。