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[class.virtual]

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  ### Virtual functions <a id="class.virtual">[[class.virtual]]</a>
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  A non-static member function is a *virtual function* if it is first
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  declared with the keyword `virtual` or if it overrides a virtual member
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- function declared in a base class (see below).[^7]
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  [*Note 1*: Virtual functions support dynamic binding and
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  object-oriented programming. — *end note*]
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  A class with a virtual member function is called a
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- *polymorphic class*.[^8]
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  If a virtual member function F is declared in a class B, and, in a class
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  D derived (directly or indirectly) from B, a declaration of a member
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  function G corresponds [[basic.scope.scope]] to a declaration of F,
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- ignoring trailing *requires-clause*s, then G *overrides*[^9]
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- F. For convenience we say that any virtual function overrides itself. A
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  virtual member function V of a class object S is a *final overrider*
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  unless the most derived class [[intro.object]] of which S is a base
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  class subobject (if any) has another member function that overrides V.
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  In a derived class, if a virtual member function of a base class
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- subobject has more than one final overrider the program is ill-formed.
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  [*Example 1*:
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  ``` cpp
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  struct A {
@@ -140,11 +140,11 @@ the return type of the overridden function or *covariant* with the
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  classes of the functions. If a function `D::f` overrides a function
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  `B::f`, the return types of the functions are covariant if they satisfy
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  the following criteria:
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  - both are pointers to classes, both are lvalue references to classes,
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- or both are rvalue references to classes[^10]
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  - the class in the return type of `B::f` is the same class as the class
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  in the return type of `D::f`, or is an unambiguous and accessible
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  direct or indirect base class of the class in the return type of
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  `D::f`
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  - both pointers or references have the same cv-qualification and the
 
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  ### Virtual functions <a id="class.virtual">[[class.virtual]]</a>
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  A non-static member function is a *virtual function* if it is first
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  declared with the keyword `virtual` or if it overrides a virtual member
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+ function declared in a base class (see below).[^5]
6
 
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  [*Note 1*: Virtual functions support dynamic binding and
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  object-oriented programming. — *end note*]
9
 
10
  A class with a virtual member function is called a
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+ *polymorphic class*.[^6]
12
 
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  If a virtual member function F is declared in a class B, and, in a class
14
  D derived (directly or indirectly) from B, a declaration of a member
15
  function G corresponds [[basic.scope.scope]] to a declaration of F,
16
+ ignoring trailing *requires-clause*s, then G *overrides*[^7]
17
 
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+ F. For convenience, we say that any virtual function overrides itself. A
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  virtual member function V of a class object S is a *final overrider*
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  unless the most derived class [[intro.object]] of which S is a base
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  class subobject (if any) has another member function that overrides V.
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  In a derived class, if a virtual member function of a base class
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+ subobject has more than one final overrider, the program is ill-formed.
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  [*Example 1*:
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  ``` cpp
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  struct A {
 
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  classes of the functions. If a function `D::f` overrides a function
141
  `B::f`, the return types of the functions are covariant if they satisfy
142
  the following criteria:
143
 
144
  - both are pointers to classes, both are lvalue references to classes,
145
+ or both are rvalue references to classes[^8]
146
  - the class in the return type of `B::f` is the same class as the class
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  in the return type of `D::f`, or is an unambiguous and accessible
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  direct or indirect base class of the class in the return type of
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  `D::f`
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  - both pointers or references have the same cv-qualification and the