final specifier (since C++11)

 

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override (C++11)

final (C++11)

 

Specifies that a virtual function cannot be overridden in a derived class or that a class cannot be derived from.

Syntax

When applied to a member function, the identifier final appears immediately after the declarator in the syntax of a member function declaration or a member function definition inside a class definition.

When applied to a class, the identifier final appears at the beginning of the class definition, immediately after the name of the class.

declarator virt-specifier-seq(optional) pure-specifier(optional)	(1)
declarator virt-specifier-seq(optional) function-body	(2)
class-key attr(optional) class-head-name class-virt-specifier(optional) base-clause(optional)	(3)

1) In a member function declaration, final may appear in virt-specifier-seq immediately after the declarator, and before the pure-specifier, if used.

2) In a member function definition inside a class definition, final may appear in virt-specifier-seq immediately after the declarator and just before function-body.

3) In a class definition, final may appear as class-virt-specifier immediately after the name of the class, just before the colon that begins the base-clause, if used.

In the cases (1,2), virt-specifier-seq, if used, is either override or final, or final override or override final. In the case (3), the only allowed value of class-virt-specifier, if used, is final

Explanation

When used in a virtual function declaration or definition, final specifier ensures that the function is virtual and specifies that it may not be overridden by derived classes. The program is ill-formed (a compile-time error is generated) otherwise.

When used in a class definition, final specifies that this class may not appear in the base-specifier-list of another class definition (in other words, cannot be derived from). The program is ill-formed otherwise (a compile-time error is generated). final can also be used with a union definition, in which case it has no effect (other than on the outcome of std::is_final) (since C++14), since unions cannot be derived from.

final is an identifier with a special meaning when used in a member function declaration or class head. In other contexts, it is not reserved and may be used to name objects and functions.

Note

In a sequence of the following tokens:

• one of class, struct and union;
• a possibly qualified identifier;
• final;
• one of : and {,

the third token final in the sequence is always considered as a specifier instead of an identifier:

struct A;
struct A final {}; // OK, definition of struct A,
                   // not value-initialization of variable final
 
struct X
{
    struct C { constexpr operator int() { return 5; } };
    struct B final : C{}; // OK, definition of nested class B,
                          // not declaration of a bit-field member final
};

Example

Run this code

struct Base
{
    virtual void foo();
};
 
struct A : Base
{
    void foo() final; // Base::foo is overridden and A::foo is the final override
    void bar() final; // Error: bar cannot be final as it is non-virtual
};
 
struct B final : A // struct B is final
{
    void foo() override; // Error: foo cannot be overridden as it is final in A
};
 
struct C : B {}; // Error: B is final

Possible output:

main.cpp:9:10: error: 'void A::bar()' marked 'final', but is not virtual
    9 |     void bar() final; // Error: bar cannot be final as it is non-virtual
      |          ^~~
main.cpp:14:10: error: virtual function 'virtual void B::foo()' overriding final function
   14 |     void foo() override; // Error: foo cannot be overridden as it is final in A
      |          ^~~
main.cpp:8:10: note: overridden function is 'virtual void A::foo()'
    8 |     void foo() final; // Base::foo is overridden and A::foo is the final override
      |          ^~~
main.cpp:17:8: error: cannot derive from 'final' base 'B' in derived type 'C'
   17 | struct C : B // Error: B is final
      |

References

• C++11 standard (ISO/IEC 14882:2011):

• 9 Classes [class]

• 10.3 Virtual functions [class.virtual]

• C++14 standard (ISO/IEC 14882:2014):

• 9 Classes [class]

• 10.3 Virtual functions [class.virtual]

• C++17 standard (ISO/IEC 14882:2017):

• 12 Classes [class]

• 13.3 Virtual functions [class.virtual]

• C++20 standard (ISO/IEC 14882:2020):

• 11 Classes [class]

• 11.7.2 Virtual functions [class.virtual]

• C++23 standard (ISO/IEC 14882:2023):

• 11 Classes [class]

• 11.7.3 Virtual functions [class.virtual]

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 1318	C++11	a class definition which has final after the class name and an empty member specification list might make final an identifier	final is always a specifier in this case

See also

override specifier(C++11)

explicitly declares that a method overrides another method