12 Overloading [over]

12.4 Overloaded operators [over.oper]

12.4.4 Function call [over.call]

A function call operator function is a function named operator() that is a non-static member function with an arbitrary number of parameters.
It may have default arguments.
For an expression of the form where the postfix-expression is of class type, the operator function is selected by overload resolution ([over.call.object]).
If a surrogate call function for a conversion function named operator conversion-type-id is selected, the expression is interpreted as Otherwise, the expression is interpreted as
postfix-expression . operator () ( expression-list )