libusb callback_in 函数作为 C++ 中 class 的成员

Question

当在 C++ 中作为 class 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。

这里是class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 class 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;
}

我是这样称呼它的：

...
...
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 class 的成员，我该如何调用它？

Answer 1

指向 class 成员函数的指针和指向函数的指针在 C++ 中不兼容。

根据文档，回调的原型是：

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

而struct libusb_transfer有user_data字段，显然是在你设置回调的时候填写的，所以你需要一个wrapper:

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);

libusb callback_in 函数作为 C++ 中 class 的成员

libusb callback_in function as a member of a class in C++

c++

libusb-1.0