final specifier (since C++11)
Specifies that a virtual function cannot be overridden in a derived class or that a class cannot be inherited from.
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) | ||||||||
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 (which may begin with a member initializer list)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
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 ofstd::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.
Example
struct Base
{
virtual void foo();
}; struct A : Base
{
virtual void foo() final; // A::foo is final
void bar() final; // Error: non-virtual function cannot be final
}; struct B final : A // struct B is final
{
void foo(); // Error: foo cannot be overridden as it's final in A
}; struct C : B // Error: B is final
{
};
See also
- override specifier (since C++11)
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