The very basics

With the exception of source code comments, everything in this section is required knowledge for anyone who intends to successfully compile a D program.

Identifiers

The names of variables, functions, user-defined types, and so on, are all identifiers. Identifiers are case-sensitive and can consist of any combination and number of Universal Character Names (UCN), underscores, and digits. D does not itself define what constitutes a valid UCN. Instead, it refers to the list of valid UCNs specified in Annex D of the C99 standard. Aside from the English alphabet, characters from several languages are valid UCNs. Henceforth, I will refer to UCNs as letters. Identifiers in this book will be constrained to the English alphabet.

There are a few rules to follow when choosing identifiers:

• The first character in an identifier can only be a letter or an underscore.
• The use of two leading underscores is reserved for the compiler implementation. This is currently not enforced by the compiler...