Alright, so I didn't find the problem, but I found a solution. Instead of deriving SerialPort in a Port class, my Port class is just a wrapper for SerialPort containing a pointer to my instance of SerialPort and having all the methods I use like that :
@bool Port::open( SerialPort::OpenMode mode )
{
return port->open( mode );
}@
I don't know if this is a "good" solution, but it works.
Anyway, thanks to kuzulis and koahnig for the help ! =)