[over.unary]

A prefix unary operator function is a function named operator@ for a prefix unary-operator @ ([expr.unary.op]) that is either a non-static member function ([class.mfct]) with no non-object parameters or a non-member function with one parameter.

For a unary-expression of the form @ cast-expression, the operator function is selected by overload resolution ([over.match.oper]).

If a member function is selected, the expression is interpreted as

cast-expression . operator @ ()

Otherwise, if a non-member function is selected, the expression is interpreted as

operator @ ( cast-expression )

[Note 1:

The operators ++ and -- ([expr.pre.incr]) are described in [over.inc].

— end note]

[Note 2:

The unary and binary forms of the same operator have the same name.

Consequently, a unary operator can hide a binary operator from an enclosing scope, and vice versa.

— end note]