docs(cpp11/02): add final and override chapter (zh + en)

book/en/src/SUMMARY.md

@@ -11,6 +11,7 @@
 
 - [Type Deduction - auto and decltype](./cpp11/00-auto-and-decltype.md)
 - [Defaulted and Deleted Functions](./cpp11/01-default-and-delete.md)
+- [final and override - Explicit Control of Virtual Function Behavior](./cpp11/02-final-and-override.md)
 - [List Initialization](./cpp11/09-list-initialization.md)
 - [Delegating Constructors](./cpp11/10-delegating-constructors.md)
 - [Inherited Constructors](./cpp11/11-inherited-constructors.md)

book/en/src/cpp11/02-final-and-override.md

@@ -0,0 +1,193 @@
+<div align=right>
+
+  🌎 [中文] | [English]
+</div>
+
+[中文]: ../../cpp11/02-final-and-override.html
+[English]: ./02-final-and-override.html
+
+# final and override - Explicit Control of Virtual Function Behavior
+
+final and override are two **context-sensitive identifiers** introduced in C++11, used in virtual-function inheritance to explicitly express the intent of **overriding** and **sealing**, allowing the compiler to surface polymorphism mismatches at compile time that would otherwise only show up as runtime bugs.
+
+| Book | Video | Code | X |
+| --- | --- | --- | --- |
+| [cppreference-final](https://en.cppreference.com/w/cpp/language/final) / [cppreference-override](https://en.cppreference.com/w/cpp/language/override) / [markdown](https://github.com/mcpp-community/d2mcpp/blob/main/book/en/src/cpp11/02-final-and-override.md) | [Video Explanation]() | [Practice Code](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp11/02-final-and-override-0.cpp) |  |
+
+
+**Why were they introduced?**
+
+- Before C++11, whether a derived class actually overrode a base virtual function relied entirely on programmers checking signatures by hand — a single mismatched parameter would silently turn an override into a name-hiding declaration with no compiler warning
+- There was no standard way to express the design intent that "this type, or this polymorphic chain, ends here"
+- They make the design contract of virtual functions readable and verifiable
+
+**What's the difference between the two?**
+
+- override: applied after a derived-class member function, explicitly declaring "this function overrides a base-class virtual function", so the compiler can verify it
+- final: applied after a virtual function means "this virtual function cannot be further overridden"; applied after a class means "this class cannot be inherited from"
+
+## I. Basic Usage and Scenarios
+
+### override - Explicitly Declare an Override
+
+Without override, even a typo in the derived signature only becomes "a brand-new ordinary function" — the polymorphic behavior is silently lost.
+
+```cpp
+struct Base {
+    virtual void func(int) { }
+};
+
+struct Derived : Base {
+    void func(double) { } // intended to override, but the parameter type is wrong;
+                          // this actually declares a new function
+};
+```
+
+With override, the same mistake is rejected at compile time.
+
+```cpp
+struct Derived : Base {
+    void func(double) override; // error: no matching virtual function in any base class
+};
+```
+
+Only a base virtual function whose signature (return type + parameter list + cv-qualifiers + ref-qualifiers) matches exactly will satisfy override.
+
+```cpp
+struct Base {
+    virtual void func(int);
+};
+
+struct Derived : Base {
+    void func(int) override; // ok
+};
+```
+
+### final - Forbid Further Overriding or Inheritance
+
+final has two usages targeting different things.
+
+**On a virtual function - cut off the polymorphic chain**
+
+```cpp
+struct A {
+    virtual void func() final { }
+};
+
+struct B : A {
+    void func() override; // error: A::func is final and cannot be overridden
+};
+```
+
+**On a class - forbid inheritance**
+
+```cpp
+struct B final { };
+
+struct C : B { }; // error: B is final and cannot be inherited from
+```
+
+### final + Pure Virtual - Non-Overridable Template Method (NVI)
+
+Lock the outer interface with `virtual ... final`, and expose the customizable steps as pure virtual functions. The result is a stable interface where **the execution order cannot be changed but each step is customizable**. This is a concise expression of the **Non-Virtual Interface** idiom.
+
+```cpp
+struct AudioPlayer {
+    virtual void play() final {  // subclasses cannot change the overall flow of play
+        init_audio_params();
+        play_audio();
+    }
+private:
+    virtual void init_audio_params() = 0; // left for subclasses to customize
+    virtual void play_audio() = 0;
+};
+
+struct WAVPlayer : AudioPlayer {
+    void init_audio_params() override { /* ... */ }
+    void play_audio() override { /* ... */ }
+};
+
+struct MP3Player : AudioPlayer {
+    void init_audio_params() override { /* ... */ }
+    void play_audio() override { /* ... */ }
+};
+```
+
+Callers always use the unified `AudioPlayer::play()`; each format's player only needs to implement the two hooks. This structure is common when designing plugin-style or protocol-style interfaces.
+
+### Context-Sensitive Identifiers
+
+Neither override nor final is a reserved word or a keyword — they are **context-sensitive identifiers**. They only carry these meanings when they appear at specific positions in a virtual function declaration or a class declaration; in any other position they can still be used as variable names, type names, namespace names, etc.
+
+```cpp
+B override; // ok: here override is just an ordinary variable name
+B final;    // ok: here final is just an ordinary variable name
+```
+
+This is a deliberate compromise in the C++ standard for backward compatibility: existing code that uses override or final as identifiers won't fail to compile after upgrading to C++11.
+
+## II. Important Notes
+
+### override Requires a Signature-Matching Base Virtual Function
+
+Once override is added to a derived-class member function, the compiler requires **a virtual function with a matching signature** to exist in some base class — otherwise it's a compile error. This is the core value of override: lifting "override mismatch" silent bugs from runtime to compile time.
+
+```cpp
+struct A {
+    virtual void func1() { }
+    void func2() { } // note: not virtual
+};
+
+struct B : A {
+    void func1() override; // ok
+    void func2() override; // error: A::func2 is not virtual
+};
+```
+
+### A final Class Is "Sealed" - Use With Care
+
+A final class cannot be inherited from at all, not even to add a couple of helper methods. Marking a class final is essentially committing to "this type is, by design, a leaf node". Some rules of thumb:
+
+- The type is explicitly not meant to be extended further (e.g. error types, framework-internal implementation classes, singletons) -> a good fit for final
+- A general-purpose base class or a framework-provided extension point -> do not casually add final
+
+### final Only Applies to Virtual Functions
+
+An ordinary member function cannot be overridden in the first place, so adding final to it is meaningless and the compiler will reject it.
+
+```cpp
+struct A {
+    void func() final; // error: final cannot be applied to a non-virtual function
+};
+```
+
+### override and final Can Be Used Together
+
+If a virtual function should both override the base-class version and forbid further overriding in derived classes, you can combine the two.
+
+```cpp
+struct B : A {
+    void func() override final; // overrides A::func, and prevents C from overriding it again
+};
+```
+
+## III. Practice Code
+
+### Practice Topics
+
+- 0 - [Familiarize with override](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp11/02-final-and-override-0.cpp)
+- 1 - [Familiarize with final](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp11/02-final-and-override-1.cpp)
+- 2 - [final + Template Method - AudioPlayer / WAV / MP3 / OGG](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp11/02-final-and-override-2.cpp)
+
+### Practice Code Auto-detection Command
+
+```
+d2x checker final-and-override
+```
+
+## IV. Additional Resources
+
+- [Discussion Forum](https://forum.d2learn.org/category/20)
+- [d2mcpp Tutorial Repository](https://github.com/mcpp-community/d2mcpp)
+- [Tutorial Video List](https://space.bilibili.com/65858958/lists/5208246)
+- [Tutorial Support Tool - xlings](https://github.com/d2learn/xlings)

book/src/SUMMARY.md

@@ -11,6 +11,7 @@
 
 - [auto和decltype - 类型自动推导](./cpp11/00-auto-and-decltype.md)
 - [...](./cpp11/01-default-and-delete.md)
+- [final 和 override - 虚函数重写控制](./cpp11/02-final-and-override.md)
 - [列表初始化](./cpp11/09-list-initialization.md)
 - [委托构造函数](./cpp11/10-delegating-constructors.md)
 - [继承构造函数](./cpp11/11-inherited-constructors.md)