c++ - libusb callback_in 函数作为 C++ 中类的成员

Question

当作为 C++ 类的成员调用时，我在 libusb_fill_bulk_transfer 中定义和使用回调函数时遇到问题。

这是类中的定义:

namespace usb_connector
{
    class USBConnector
    {
    public:
        USBConnector();
        ~USBConnector();
        int connect(void);
        void read(void);
        void write(unsigned char *);
        void disconnect(void);
        void LIBUSB_CALL callback_in(struct libusb_transfer*);
...

下面是类中的实现:

void LIBUSB_CALL usb_connector::USBConnector::callback_in(struct libusb_transfer *transfer)
{
    if (transfer == NULL) {
        cout << "No libusb_transfer..." << endl;
    }
    else {
        cout << "libusb_transfer structure: " << endl;
        cout << "actual_length = " << transfer->actual_length << endl;
        for (int i = 0; i < transfer->actual_length; i++) {
            cout << transfer->buffer[i];
        }
        cout << endl;
    }

    return;
}

我是这样调用它的:

...
...
libusb_fill_bulk_transfer( transfer_in, usb_dev, USB_ENDPOINT_IN,
            in_buffer,  LEN_IN_BUFFER, callback_in, NULL, 0);
...
...

我得到的错误如下:

error: cannot convert 'usb_connector::USBConnector::callback_in' from type 'void (usb_connector::USBConnector::)(libusb_transfer*)' to type 'libusb_transfer_cb_fn {aka void ()(libusb_transfer)}' in_buffer, LEN_IN_BUFFER, callback_in, NULL, 0);

如何将回调函数作为类的成员以及如何调用它？

Answer 1

在 C++ 中，指向类成员函数的指针和指向函数的指针是不兼容的。

根据文档，回调的原型(prototype)是:

typedef void( * libusb_transfer_cb_fn) (struct libusb_transfer *transfer)

而 struct libusb_transfer 有 user_data 字段，这显然是在你设置回调时填写的，所以你需要一个包装器:

void LIBUSB_CALL callback_wrapper(struct libusb_transfer *transfer)
{
    usb_connector::USBConnector *connector = reinterpret_cast<usb_connector::USBConnector*>(transfer->user_data);
    connector->callback_in(transfer);
}

并在调用过程中传递this:

...
...
libusb_fill_bulk_transfer( transfer_in, usb_dev, USB_ENDPOINT_IN,
            in_buffer,  LEN_IN_BUFFER, callback_wrapper, this, 0);