Always give any new class a default constructor, a copy constructor, and an assignment operator.
Another misconception I see often is a fuzzy idea of the difference between the copy constructor and the assignment operator.
These three functions are special in C : If you don't provide them yourself, C provides them for you. Among other things, this means you have to define these operations even if you don't want a client to be able to copy or default-construct a particular class.
If you don't want a class to be copied, for example, you have to define an empty copy constructor and assignment operator yourself and make them private or protected.
The copy constructor is creating a new object from raw storage and knows it's writing over garbage.
For many classes, the current state of the object doesn't matter and both functions do the same thing.
Unfortunately, we would have rated almost all of them somewhere between a 4 and a 6.
In my opinion, this goes to show you that working with C for a long time doesn't guarantee you really understand the language.
Over this time, I've developed a stock interview question that's proven to be a pretty good gauge of C knowledge.
No one has yet been able to just rip out the correct answer, but we've had several, including the guy we hired, who understood the important issues and were able to get the question right with prompting.