how to get vendor id and product id of a plugged usb device on windows

After this line:

SP_DEVICE_INTERFACE_DETAIL_DATA *pDetData = NULL;

Add this:

DWORD dwDetDataSize = sizeof (SP_DEVICE_INTERFACE_DETAIL_DATA) + 256;
pDetData = (_SP_DEVICE_INTERFACE_DETAIL_DATA_A*) malloc (dwDetDataSize);
pDetData->cbSize = sizeof (SP_DEVICE_INTERFACE_DETAIL_DATA);

After this line:

qDebug ()<<pDetData->DevicePath;

Add this:

free(pDetData);

But eventually you're going to have to read the docs for SetupDiGetDeviceInterfaceDetail(). Do it, there are lots of functions that work like this, with pointers to variable-size structs.

-------- Edited to add: --------

You're really going about this the wrong way. I see you're following the advice you got here, and it's taken you down the wrong path. idVendor and idProduct can only be found in the USB_DEVICE_DESCRIPTOR (MSDN).

It looks like you already know how to get the device handle (using CreateFile()). After that, you call WinUsb_Initialize() (MSDN). That gets you a WINUSB_INTERFACE_HANDLE.

Once you have that handle, you want to call WinUsb_GetDescriptor() (MSDN), with the DescriptorType set to URB_FUNCTION_GET_DESCRIPTOR_FROM_DEVICE. I can't test code now, but it will look something like this:

USB_DEVICE_DESCRIPTOR udd;
memset(&udd, 0, sizeof(udd));
ULONG LengthTransferred = 0;

WinUsb_GetDescriptor(
    winusb_interface_handle, // returned by WinUsbInitialize
    URB_FUNCTION_GET_DESCRIPTOR_FROM_DEVICE,
    0,     // not sure if we need this
    0x409, // not sure if we need this
    &udd,
    sizeof(udd),
    &LengthTransferred);

After that, udd->idVendor and udd->idProduct should have what you want.

Microsoft used to supply sample code for all this in the DDK, and probably still does, but I don't have access to one.

---------- Edited to add: ----------

Daniel K writes that the code should really be:

USB_DEVICE_DESCRIPTOR udd;
memset(&udd, 0, sizeof(udd));
ULONG LengthTransferred = 0;

WinUsb_GetDescriptor(
    winusb_interface_handle,    // returned by WinUsbInitialize
    USB_DEVICE_DESCRIPTOR_TYPE, // Daniel K's suggestion
    0,
    0x409,     // asks for English
    &udd,
    sizeof(udd),
    &LengthTransferred);

See the comments for further details.