final specifier (since C++11)

 

C++

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Technical Specifications

 

C++ language

 

Classes

General

overview

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Members

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override(C++11)

final(C++11)

pure virtual functions and abstract classes

 

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)

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

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.

Example

struct A
{
    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)