final specifier (since C++11)
Specifies that a virtual function cannot be overridden in a derived class, or that a class cannot be derived from.
Contents |
[edit] 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 (including struct and union), the identifier final appears at the beginning of the class definition, immediately after the name of the class, and cannot appear in a class declaration.
| 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) | ||||||||
final may appear in virt-specifier-seq immediately after the declarator, and before the pure-specifier, if used.final may appear in virt-specifier-seq immediately after the declarator and just before function-body.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.
[edit] 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.
[edit] 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 }; // 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() { }
[edit] Keywords
[edit] Example
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
|[edit] References
- C++23 standard (ISO/IEC 14882:2024):
- 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]
[edit] 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 |
[edit] See also
override specifier(C++11)
|
explicitly declares that a method overrides another method |
