Visual Studio 2008: String Literals "??-", "??'", "??=" corrupt

Question

Recently I came across a bug in Visual Studio 2008 (at least I think it is one).

When I try to create string-literals with two questionmarks followed by another character, something weird occurs: Those three chars are replaced by another char.

Examples:

printf("??-"); --> ~  (hyphen)
printf("??'"); --> ^  (circumflex)
printf("??="); --> #  (hash)
printf("??)"); --> ]  (square braket)
printf("??("); --> [  (square braket)
printf("??/"); --> \  (backslash)
printf("??!"); --> |  (pipe)
printf("??%"); --> ?? (percent sign disappears)

Does anybody know the reason for this replacement?

Answer 1

It's no bug, more a hangover of history. They're C trigraphs - https://msdn.microsoft.com/en-us/library/bt0y4awe.aspx

Di/Tri-graphs are a way to represent characters that weren't easily available back when the language was developed. They let you enter symbols into your source code that you may not have a keyboard key to represent.

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

The % issue is just the usual one in printf where you need to escape a % with a % if you wish to see it. This is because the % is usually the beginning of a format specifier.