A binary operator shall be implemented either by a non-static member
function (
[class.mfct])
with one parameter or by a non-member function with two parameters
.Thus, for any binary operator
@,
x@y
can be interpreted as either
x.operator@(y)
or
operator@(x,y).If both forms of the operator function have been declared,
the rules in
[over.match.oper] determine which, if any, interpretation is
used
.