If you know C well then the switch to C++ shouldn't be too hard.
I have been learning C++ for three years and learnt some C at school. There is still much of the language I do not know and am still learning new bits and peices each day. As with C, id say the hardest topics to grasp is memory management, ie: pointers (malloc() ) in C and file reading was a little difficult to grasp I find too.
If you are a dedicated C programmer and are looking to switch to C++ then prepeare to see the C langauge stretched and altered in ways you never thought possible