cc-mode有如下规定：
One of the symbols +, -, ++, --, *, or /These special symbols describe a relative offset in multiples of c-basic-offset:
By defining a style's indentation in terms of c-basic-offset, you can change the amount of whitespace given to an indentation level while maintaining the same basic shape of your code. Here are the values that the special symbols correspond to:
+ c-basic-offset times 1
- c-basic-offset times -1
++ c-basic-offset times 2
-- c-basic-offset times -2
* c-basic-offset times 0.5
/ c-basic-offset times -0.5