libusb callback_in函數作為C ++中類的成員

Question

當在C ++中作為類的成員調用時，我在libusb_fill_bulk_transfer定義和使用回調函數時libusb_fill_bulk_transfer問題。

這是類中的定義：

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*);
...

這是類中的實現：

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;
}

以下是我稱之為：

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

我得到的錯誤如下：

錯誤：無法將'usb_connector :: USBConnector :: callback_in'從類型'void（usb_connector :: USBConnector ::）（libusb_transfer *）'轉換為'libusb_transfer_cb_fn {aka void（ ）（libusb_transfer ）}'in_buffer，LEN_IN_BUFFER，callback_in， NULL，0）;

如何將回調函數作為類的成員以及如何調用它？

Answer 1

指向類成員函數的指針和指向函數的指針在C ++中是不兼容的。

根據文檔，回調的原型是：

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

而struct libusb_transfer有user_data字段，顯然在你設置回調期間填寫，所以你需要一個包裝器：

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);
}

並通過this通話過程中：

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