final specifier (since C++11)

< cpp‎ | language

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


[edit] Syntax

The identifier final, if used, appears immediately after the declarator in the syntax of a member function declaration or a member function definition.

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-specifier-list(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, final may appear in virt-specifier-seq immediately after the declarator and just before function-body (which may begin with a member initializer list)
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-specifier-list, 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 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 (a compile-time error is generated) otherwise. 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 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] Example

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

[edit] See also