Exceptions

When any of the special functions are declared only to make them private or virtual, but without special semantics, it doesn't imply that the others are needed.

In rare cases, classes that have members of strange types (e.g., references, std::auto_ptrs) are an exception because they have peculiar copy semantics. In a class holding a reference or an auto_ptr, you likely need to write the copy constructor and the assignment operator, but the default destructor already does the right thing. (Note that using a reference or auto_ptr member is almost always wrong.)