请支持原创~~
1. 功能
- 用以指定一个 virtual function 不能被派生类重写;
- 或者指定一个 class 不能被继承;
2. 语法
对于类中成员函数有两种情况:
- 只声明时,final 紧跟参数的右括号,如果是纯虚函数,final 会出现在 = 0 之前;
- 类中定义时,final 在 函数体之前;
对于类定义,final 紧跟类名;
从c++14 开始,会引入一个 is_final 的判断,确认一个 class 是否被定义为 final,通过使用:
std::is_final<A>::value
来判断 class A 是否为 final。
3. 举例
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()
{
}
final 只是一个标识,在使用成员函数和类定义时有特殊的意义。而在其他情况下可以作为一个对象名、函数名、类名使用。
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
注意这里的几个注意点:
- A 类中 foo() 继承自 Base 类,并把自身定义为 final,即后续继承A 的子类无法再重写 foo();
- A 类中 bar() 使用了final,但 final 是用在 virtual 的成员函数;
- B 类想重写 foo(),是不允许的,因为在A类中已经声明为 final;
- B 类定义的时候已经被声明为 final,所以无法再被其他类继承,即B 类不会有子类;
4. 原文摘录
Specifies that a virtual function cannot be overridden in a derived class or that a class cannot be derived from.
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.
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
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.