Purpose of Trigraph sequences in C++?
Kids today! :-)
Yes, foreign equipment, such as an IBM 3270 terminal. The 3270 has, if I remember, no curly braces! If you wanted to write C on an IBM mini / mainframe, you had to use the wretched trigraphs for every block boundary. Fortunately, I only had to write software in C to emulate some IBM minicomputer facilities, not actually write C software on the System/36.
Look next to the "P" key:
Hmmm. Hard to tell. There is an extra button next to "carriage return", and I might have it backwards: maybe it was the "[" / "]" pair that was missing. At any rate, this keyboard would cause you grief if you had to write C.
Also, these terminals display EBCDIC, IBM's "native" mainframe character set, not ASCII (thanks, Pavel Minaev, for the reminder).
On the other hand, like the GNU C guide says: "You don't need this brain damage." The gcc compiler leaves this "feature" disabled by default.
This question (about the closely related digraphs) has the answer.
It boils down to the fact that the ISO 646 character set doesn't have all the characters of the C syntax, so there are some systems with keyboards and displays that can't deal with the characters (though I imagine that these are quite rare nowadays).
In general, you don't need to use them, but you need to know about them for exactly the problem you ran into. Trigraphs are the reason the the '?' character has an escape sequence:
'\?'
So a couple ways you can avoid your example problem are:
printf( "What?\?!\n" );
printf( "What?" "?!\n" );
But you have to remember when you're typing the two '?' characters that you might be starting a trigraph (and it's certainly never something I'm thinking about).
In practice, trigraphs and digraphs are something I don't worry about at all on a day-to-day basis. But you should be aware of them because once every couple years you'll run into a bug related to them (and you'll spend the rest of the day cursing their existance). It would be nice if compilers could be configured to warn (or error) when it comes across a trigraph or digraph, so I could know I've got something I should knowingly deal with.
And just for completeness, digraphs are much less dangerous since they get processed as tokens, so a digraph inside a string literal won't get interpreted as a digraph.
For a nice education on various fun with punctuation in C/C++ programs (including a trigraph bug that would defintinely have me pulling my hair out), take a look at Herb Sutter's GOTW #86 article.
Addendum:
It looks like GCC will not process (and will warn about) trigraphs by default. Some other compilers have options to turn off trigraph support (IBM's for example). Microsoft started supporting a warning (C4837) in VS2008 that must be explicitly enabled (using -Wall or something).