[class.mfct.non.static]

A non-static member function may be called for an object of its class type, or for an object of a class derived ([class.derived]) from its class type, using the class member access syntax ([expr.ref], [over.match.call]).

A non-static member function may also be called directly using the function call syntax ([expr.call], [over.match.call]) from within its class or a class derived from its class, or a member thereof, as described below.

[Note 1:

An implicit object member function can be declared with cv-qualifiers, which affect the type of the this pointer ([expr.prim.this]), and/or a ref-qualifier ([dcl.fct]); both affect overload resolution ([over.match.funcs]).

— end note]

An implicit object member function may be declared virtual ([class.virtual]) or pure virtual ([class.abstract]).