Dividing sketch (classes)

So, in my main tab:

void setup()
{
sample_class test;
}

void loop()
{}

and in another tab

class sample_class
{
};

What includes do I need? This thread seemed to suggest that none were required as the compiler treated them as a single file? Unless I've misinterpreted it: http://forum.arduino.cc/index.php/topic,41039.0.html

Thanks in advance