错误:无效使用非静态成员函数“int test::hotplug_callback(libusb_context*，libusb_device*，libusb_hotplug_event，void*)”

Question

我修改了libusb热插拔示例，该示例来自

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

(下面的代码)作为一个测试，并将它放在一个类中，因为我的实际程序有相同的问题。我知道，如果我把静态放在两个回调的前面，那么它是起作用的，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。我们的想法是，用户在usb设备中插入，让我们称其为usbXYZ。在回调中，我实例化类usbXYZ并放入std::map - user删除它，然后从映射中删除它。包含std::map及其中的对象的类有一个“高级”方法来写入设备。

如何使回调函数不静态地工作？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)指针与非静态成员函数指针之间的区别是，非静态成员函数需要调用对象，而C不知道C++“对象”。

使用libusb，您可以使用static成员函数和用户数据指针(现在传递一个空指针)来解决这个问题。

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static成员函数，并将this作为用户数据指针传递。