Modern C++ has many additions compared to the original C++ standard. Regarding virtual overrides, C++11 tends to tighten the rules, to detect some problems that often arise. To achieve this goal C++11 introduces two new contextual keywords, the final and the override. The final specifier (keyword) is used for a function or for a class that cannot be derived and overridden by derived classes. This final specifier is used with the C++ compiler that has C++11 and the other higher C++ standards. In this post, we explain how to use the final specifier in modern C++.
What Is the final specifier in C++?
The final specifier (keyword) is used for a function or for a class that cannot be overridden by derived classes. Regarding virtual overrides, C++11 tends to tighten the rules, to detect some problems that often arise.
To achieve this goal C++11 introduces two new contextual keywords:
- final specifies that a method cannot be overridden, or a class cannot be derived.
- override specifies that a method overrides a virtual method declared in one of its parent classes.
In this post, we explain how to use the final specifier in C++.
The final specifier is and Explicit Virtual Override that prevents a class from being further inherited or prevents a function from being overridden. We can add the final specifier to a class definition or to a virtual member function declaration inside a class definition.
A class with the final specifier is not allowed to be a base class for another class. A virtual function with the final specifier cannot be overridden in a derived class. If a virtual member function f in some class B is marked final and in a class D derived from B, a function D::f overrides B::f, the program is ill-formed (the compiler does not issue a message).
How to use the final specifier in C++ functions?
The final specifier is used to designate virtual functions that cannot be overridden in a derived class. Here are the syntaxes of how to use it:
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function_declaration final; |
or with the body,
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function_declaration final { } |
or with virtual function declaration,
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virtual function_declaration final; |
and, here is an example for the Clang-enhanced C++ compilers
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#include <iostream> class Tx { virtual void f() final; }; class Ty : public Tx { virtual void f(); // ERROR: declaration of 'f' overrides a 'final' function }; int main() { Ty o1; return 0; } |
in previous-generation compilers, it was used with the [[final]] keyword like this:
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#include <iostream> class Tx { virtual void f() [[final]]; }; class Ty : public Tx { virtual void f(); // ERROR: declaration of 'f' overrides a 'final' function }; int main() { Ty o1; return 0; } |
If you need an official docwiki, please check this: Workaround for C++11 attributes
How to use the final specifier in modern C++ classes?
The final specifier is used to designate classes that cannot be inherited. Here are the syntaxes about how to use it:
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class class_name final |
or with other base classes as below,
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class class_name final base_classes |
this is how we can use it:
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#include <iostream> class Ta final { }; class Tb : Ta // ERROR: base 'ta' is marked 'final' { }; int main() { Tb o1; return 0; } |
For more information on this feature, see Explicit virtual overrides Proposal document.
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