11 Classes [class]

11.4 Class members [class.mem]

11.4.3 Non-static member functions [class.mfct.non.static]

1
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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.
2
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[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]
3
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An implicit object member function may be declared virtual ([class.virtual]) or pure virtual ([class.abstract]).