What is the meaning of `???-` in C++ code?

It's actually:

size_t a = 1 ? ~2 :0;

??- is a trigraph for ~

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Trigraphs are from an old era... before some of us were even born.

Back in the days, there were some characters that weren't always supported. An unknowing programmer would try to type in such a character only to find that it doesn't exist on the keyboard!

_{Image Source: http://www.myoldmac.net/cgi-data/forum/phpBB2/viewtopic.php?t=305}

So trigraphs were added to allow the programmer to access the functionality of these characters when they didn't exist (either in the encoding or from the keyboard).

Nowadays, they are obsolete and are more effective in confusing the reader than in getting around old standards.

So either that code is really old, or the author was being a jerk.

??- is a trigraph for the tilde ~ character; the line is equivalent to:

size_t a = 1 ? ~2 :0;