Cryptic line "??!??!" in legacy code

Question

I am refactoring some very old legacy code which is full of bugs and very questionable practices, at least for modern standards. Now I ran across one line which I simply cannot decipher:

p and k are of type int *

return p??!??!k?p?*p:sizeof(*k):0;

When I saw it I could not believe my eyes - I know the ? operator, but its syntax is bool ? trueresult : falseresult and a ?? operator does neither make sense (lazy evaluation does really not apply here), not could I find a reference of that mysterious operator anywhere.

It would be really cool if someone shed some light on this matter.

Which compiler are you using? gcc, for example, ignores trigraphs by default. And yes, this must be *very, very old* code, indeed. — devnull, Jul 18 '13 at 08:29
@devnull It's gcc but it's compiled with a Makefile. And now that I know what trigraphs are I also know why there is a `-trigraphs` flag passed to the compiler ;) — user2573221, Jul 18 '13 at 08:32
Trigraphs (`??!`) are evil; chained ternary operators (`??::`) are also evil; mixing them both in the same line is too much! — rodrigo, Jul 18 '13 at 08:52
This is pure garbage. I'm now going to the duplicate question to add this comment there, too. — Daniel Daranas, Jul 18 '13 at 09:51
Read it as "Either no ErrorHasOcurred or you must HandleError" — Prakhar Agarwal, Jan 03 '18 at 03:16

Yu Hao · Accepted Answer · 2013-07-18T08:40:35.817

It's called Trigraph:

C11(ISO/IEC 9899:201x) §5.2.1.1 Trigraph sequences

Before any other processing takes place, each occurrence of one of the following sequences of three characters (called trigraph sequences17)) is replaced with the corresponding single character.

??=    #
??(    [
??/    \
??)    ]
??'    ^
??<    {
??!    |
??>    }
??-    ~

It's also in C++11(ISO/IEC 14882:2011) § 2.3 Trigraph sequences

So after trigraph replacement, the line return p??!??!k?p?*p:sizeof(*k):0; turns into

return p || k ? p ? *p : sizeof(*k) : 0

Since the ternary operator has a rather low precedence, it's actually:

return (p || k) ? (p ? (*p) : sizeof(*k)) : 0;

user123 · Answer 2 · 2013-07-18T08:33:15.940

8

That line of code is equivalent to:

return p || k? p? *p : sizeof(*k) : 0;

Or more clearly:

return (p || k)? (p? (*p) : sizeof(*k)) : 0;

edited Jul 18 '13 at 08:33

answered Jul 18 '13 at 08:28

user123

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Cryptic line "??!??!" in legacy code

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