# `final` specifier (since C++11)

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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:

Run this code

    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
    };

    // Abnormal final usage.

    struct final final // OK, definition of a struct named `final` from which
    {                  // you cannot inherit
    };

    // struct final final {}; // Error: redefinition of `struct final`, NOT a
                              // definition of a variable `final` using an elaborated
                              // type specifier `struct final` followed by an
                              // aggregate initialization

    // struct override : final {}; // Error: cannot derive from final base type;
                                   // `override` in given context is a normal name
    void foo()
    {
        [[maybe_unused]]
        final final; // OK, declaration of a variable named `final` of type
                     // `struct final`
    }

    struct final final; // OK, declaration of a variable named `final` of type
                        // `struct final` using an elaborated type specifier
    int main()
    {
    }

### 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++23 standard (ISO/IEC 14882:2023):

  - 11 Classes [class]

  - 11.7.3 Virtual functions [class.virtual]

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

  - 11 Classes [class]

  - 11.7.2 Virtual functions [class.virtual]

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

  - 12 Classes [class]

  - 13.3 Virtual functions [class.virtual]

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

  - 9 Classes [class]

  - 10.3 Virtual functions [class.virtual]

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

  - 9 Classes [class]

  - 10.3 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

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