The use of tri and di-graphs isn't written in this day, it exists only in very old code that was created in a very limited environment. Any code that contains trigraphs, if you attempt to compile them on a modern compiler like VS's,it will usually not compile unless you specify a linker option. I know for Visual Studio, that option is "/Zc:trigraphs"
Why they exist, is because the C++ committee never issues changes that would 'break' legacy code. For better or for worse. There is an anecdote that their removal was proposed and supported, and it was stopped by a lone IBM representative.
