Let me clear up a common misconception. When most people say writing in code, what they really mean is writing using a cipher. A code is when a symbol, word, or phrase is used to stand for a different word, phrase, or sentence. A cipher is were each letter in a word is represented with a different letter or symbol.
The dash is a punctuation mark that is similar to a hyphen or minus sign, but differs from both of these symbols primarily in length and function.
A double hyphen is a dash. That is, when you put a double dash in a manuscript, a printer will transpose it into a longer em dash. It is often used in pairs, so it is the equivalent of parentheses, or even commas around a clause or phrase. The double dash is also called an em dash.. It has recognized uses, but mostly, as I say above, in place of parentheses. Here is what Chicago says about the em or double dash:6.87Versatility and frequency of use The em dash, often simply called the dash, is the most commonly used and most versatile of the dashes. To avoid confusion, no sentence should contain more than two em dashes; if more than two elements need to be set off, use parentheses