Now that you understand the basics of the reference in C++, you’re ready to tackle one of the more confusing concepts in the language: the copy-constructor, often called X(X&) (“X of X ref”). This constructor is essential to control passing and returning of user-defined types by value during function calls. It’s so important, in fact, that the compiler will automatically synthesize a copy-constructor if you don’t provide one yourself, as you will see.