以下摘自*，mark一下，以备不时之需。
For the purposes of this table, a, b, and c represent valid values (literals, values from variables, or return value), object names, or lvalues, as appropriate. R, S and T stand for any type(s), and K for a class type or enumerated type.
"Can overload" means that the operator can be overloaded in C++. "Included in C" means that the operator exists and has a semantic meaning in C (operators are not overloadable in C).
