Item 12: Copy all parts of an object

In well-designed object-oriented systems that encapsulate the internal parts of objects, only two functions copy objects: the aptly named copy constructor and copy assignment operator. We'll call these the copying functions. Item 5 observes that compilers will generate the copying functions, if needed, and it explains that the compiler-generated versions do precisely what you'd expect: they copy all the data of the object being copied.

When you declare your own copying functions, you are indicating to compilers that there is something about the default implementations you don't like. Compilers seem to take offense at this, and they retaliate in a curious fashion: they don't tell you when your implementations are almost certainly wrong.