docs(cpp11): add STL real-world case, exercise mapping, and tagged-union exercise for 16-generalized-unions (mcpp-community#57)

* docs(cpp11): add STL real-world case and exercise mapping for 16-generalized-unions

- add section 二 真实案例 — include std::variant _Variant_storage_ and
  std::any _Storage_t examples, both cited from vendored msvc-stl/

- fill in 四 练习代码 — exercise descriptions, file links and
  d2x checker command

- fix empty Code link in header table

Case selection notes:
  Picked _Variant_storage_ over std::optional _Optional_destruct_base
  because the variant recursive union demonstrates both key C++11
  generalized-union capabilities — non-trivial members and manual
  destructor management — while the optional pattern is effectively
  a special case of variant. std::any adds a second typical use:
  union as type-erased storage. Both are directly traceable in the
  vendored msvc-stl/, consistent with how ch00 cites xutility.

* docs(cpp11): fix exercise list format to match project convention

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* docs(cpp11): apply Copilot review suggestions — remove inaccurate noexcept, fix 类型安全 wording

* feat(cpp11): add tagged-union exercise for 16-generalized-unions

Add exercise 2 — tagged/discriminated union combining enum tag + union
to implement a simplified std::variant-like Value type.

Exercise progression:
  0. union default member initialization (static rule)
  1. non-trivial type + placement new + manual destructor
  2. enum tag + union → type-safe discriminated union

Design rationale:
  ex 2 ties ex 0 and ex 1 together by adding a tag enum to track
  the active member — the exact pattern used by std::variant's
  internal _Variant_storage_, directly echoing the STL real-world
  case in the chapter. Learners construct/destruct/switch members
  based on tag, covering all three C++11 generalized-union
  capabilities in one practical exercise.

* docs(cpp11): sync en book chapter with zh for 16-generalized-unions

Apply the same updates to the English book chapter:
  - add section II Real-World Case (msvc-stl variant + any examples)
  - fill in section IV Exercise Code (exercise links + d2x checker command)
  - fix empty Code link in header table
  - add ex2 tagged-union exercise link

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: lczllx

book/en/src/cpp11/16-generalized-unions.md

@@ -15,7 +15,7 @@ The size of a union is at least large enough to hold the largest data member.
 
 | Book | Video | Code | X |
 | --- | --- | --- | --- |
-| [cppreference-union](https://cppreference.com/w/cpp/language/union.html) / [markdown](https://github.com/mcpp-community/d2mcpp/blob/main/book/src/cpp11/16-generalized-unions.md) | [Video Explanation]() | [Exercise Code]() |  |
+| [cppreference-union](https://cppreference.com/w/cpp/language/union.html) / [markdown](https://github.com/mcpp-community/d2mcpp/blob/main/book/src/cpp11/16-generalized-unions.md) | [Video Explanation]() | [Exercise Code](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp11/16-generalized-unions-0.cpp) |  |
 
 **Why introduced?**
 
@@ -127,7 +127,56 @@ int main() {
 }
 ```
 
-## II. Precautions
+## II. Real-World Case — Generalized Unions in the STL
+
+**std::variant Internal Storage — Recursive Generalized Union**
+> Using the `std::variant` implementation from the vendored [MSVC STL](https://github.com/mcpp-community/d2mcpp/tree/main/msvc-stl) as an example (source: [`msvc-stl/stl/inc/variant`](https://github.com/mcpp-community/d2mcpp/blob/main/msvc-stl/stl/inc/variant#L343-L399)), `_CONSTEXPR20` / `_STD` are internal macros and can be ignored while reading
+
+```cpp
+// MSVC STL · msvc-stl/stl/inc/variant (abridged)
+template <class _First, class... _Rest>
+class _Variant_storage_<false, _First, _Rest...> {
+public:
+    union {
+        remove_cv_t<_First> _Head;
+        _Variant_storage<_Rest...> _Tail;
+    };
+
+    _CONSTEXPR20 ~_Variant_storage_() {
+        // The union does not know which member is active; destruction is
+        // controlled by the outer variant class
+    }
+    // ...
+};
+```
+
+`std::variant` uses a recursive union to hold multiple types in a single block of memory. The non-trivial destructor variant must explicitly define the destructor — this is exactly the key capability of C++11 generalized unions: unions can contain members with non-trivial special member functions, but their lifecycle must be managed manually
+
+**std::any Small-Object Optimization — Union as Type-Erased Storage**
+> `std::any` uses a union to combine small-object storage, heap pointers, and raw byte buffers into the same memory (source: [`msvc-stl/stl/inc/any`](https://github.com/mcpp-community/d2mcpp/blob/main/msvc-stl/stl/inc/any#L362-L376))
+
+```cpp
+// MSVC STL · msvc-stl/stl/inc/any (abridged)
+class any {
+    struct _Storage_t {
+        union {
+            unsigned char _TrivialData[_Any_trivial_space_size];
+            _Small_storage_t _SmallStorage;
+            _Big_storage_t _BigStorage;
+        };
+        uintptr_t _TypeData;
+    };
+
+    union {
+        _Storage_t _Storage{};
+        max_align_t _Dummy;
+    };
+};
+```
+
+> Summary: The core storage of both `std::variant` and `std::any` relies on generalized unions. Before C++11, unions could only hold POD types, forcing the standard library to use raw byte buffers + placement new as a workaround. Generalized unions allow code to directly express "one-of-many" memory layout, with an outer wrapper responsible for tracking the active member and managing its lifecycle, making the code easier to maintain
+
+## III. Precautions
 
 **Accessibility**
 
@@ -171,11 +220,21 @@ m.a = 1;
 double c = m.b; // Error: undefined behavior.
 ```
 
-## III. Exercise Code
+## IV. Exercise Code
+
+### Exercise Code Topics
 
-TODO
+- 0 - [Union Default Member Initialization](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/en/cpp11/16-generalized-unions-0.cpp)
+- 1 - [Union with Non-Trivial Types and Lifecycle Management](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/en/cpp11/16-generalized-unions-1.cpp)
+- 2 - [Tagged Discriminated Union — A Simplified std::variant with enum + union](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/en/cpp11/16-generalized-unions-2.cpp)
+
+### Exercise Auto-Checker Command
+
+```
+d2x checker generalized-unions
+```
 
-## IV. Other
+## V. Other
 
 - [Discussion Forum](https://forum.d2learn.org/category/20)
 - [d2mcpp Tutorial Repository](https://github.com/mcpp-community/d2mcpp)

book/src/cpp11/16-generalized-unions.md

@@ -14,7 +14,7 @@
 
 | Book | Video | Code | X |
 | --- | --- | --- | --- |
-| [cppreference-union](https://cppreference.com/w/cpp/language/union.html) / [markdown](https://github.com/mcpp-community/d2mcpp/blob/main/book/src/cpp11/16-generalized-unions.md) | [视频解读]() | [练习代码]() |  |
+| [cppreference-union](https://cppreference.com/w/cpp/language/union.html) / [markdown](https://github.com/mcpp-community/d2mcpp/blob/main/book/src/cpp11/16-generalized-unions.md) | [视频解读]() | [练习代码](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp11/16-generalized-unions-0.cpp) |  |
 
 **为什么引入**
 
@@ -126,7 +126,55 @@ int main() {
 }
 ```
 
-## 二、注意事项
+## 二、真实案例 - STL 中的广义联合体
+
+**std::variant 的内部存储 - 递归广义联合体**
+> 以仓库内置的 [MSVC STL](https://github.com/mcpp-community/d2mcpp/tree/main/msvc-stl) 中的 `std::variant` 实现为例 (源码: [`msvc-stl/stl/inc/variant`](https://github.com/mcpp-community/d2mcpp/blob/main/msvc-stl/stl/inc/variant#L343-L399)), `_CONSTEXPR20` / `_STD` 是库内部宏, 阅读时可忽略
+
+```cpp
+// MSVC STL · msvc-stl/stl/inc/variant (有删节)
+template <class _First, class... _Rest>
+class _Variant_storage_<false, _First, _Rest...> {
+public:
+    union {
+        remove_cv_t<_First> _Head;
+        _Variant_storage<_Rest...> _Tail;
+    };
+
+    _CONSTEXPR20 ~_Variant_storage_() {
+        // 联合体不知道哪个成员活跃, 析构由外层 variant 控制
+    }
+    // ...
+};
+```
+
+`std::variant` 通过递归联合体在一块内存里容纳多个不同类型, 非平凡析构版本必须显式定义析构函数 — 这正是 C++11 广义联合体的核心能力: 联合体可以包含有非平凡特殊成员函数的成员, 但需要手动管理生命周期
+
+**std::any 的小对象优化 - 联合体做类型擦除存储**
+> `std::any` 使用联合体将小对象、大对象指针和原始缓冲区合并到同一块内存 (源码: [`msvc-stl/stl/inc/any`](https://github.com/mcpp-community/d2mcpp/blob/main/msvc-stl/stl/inc/any#L362-L376))
+
+```cpp
+// MSVC STL · msvc-stl/stl/inc/any (有删节)
+class any {
+    struct _Storage_t {
+        union {
+            unsigned char _TrivialData[_Any_trivial_space_size];
+            _Small_storage_t _SmallStorage;
+            _Big_storage_t _BigStorage;
+        };
+        uintptr_t _TypeData;
+    };
+
+    union {
+        _Storage_t _Storage{};
+        max_align_t _Dummy;
+    };
+};
+```
+
+> 小结: `std::variant`、`std::any` 的核心存储都依赖广义联合体。C++11 之前联合体只能容纳 POD 类型, 标准库不得不用原始字节缓冲区 + placement new 的迂回方案; 广义联合体让代码可以直接表达"多选一"的内存布局, 并由外层封装负责跟踪活跃成员与管理生命周期, 因而更易维护
+
+## 三、注意事项
 
 **可访问性**
 
@@ -170,11 +218,21 @@ m.a = 1;
 double c = m.b; // 错误：未定义行为
 ```
 
-## 三、练习代码
+## 四、练习代码
+
+### 练习代码主题
 
-TODO
+- 0 - [联合体默认成员初始化 - 最多一个变体成员可带默认初始化器](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp11/16-generalized-unions-0.cpp)
+- 1 - [联合体包含非平凡类型及生命周期管理 - placement new 构造 / 显式析构](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp11/16-generalized-unions-1.cpp)
+- 2 - [带标签的鉴别联合体 - 用 enum + union 实现简易 variant](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp11/16-generalized-unions-2.cpp)
+
+### 练习代码自动检测命令
+
+```
+d2x checker generalized-unions
+```
 
-## 四、其他
+## 五、其他
 
 - [交流讨论](https://forum.d2learn.org/category/20)
 - [d2mcpp教程仓库](https://github.com/mcpp-community/d2mcpp)

dslings/cpp11/16-generalized-unions-2.cpp

@@ -0,0 +1,105 @@
+// d2mcpp: https://github.com/mcpp-community/d2mcpp
+// license: Apache-2.0
+// file: dslings/cpp11/16-generalized-unions-2.cpp
+//
+// Exercise/练习: cpp11 | 16 - generalized unions | 带标签的鉴别联合体
+//
+// Tips/提示:
+// - 使用 enum 标记联合体中哪个成员是活跃的
+// - 根据标签决定构造哪个成员、析构哪个成员
+// - 联合体本身不知道哪个成员活跃, 标签由外层结构体维护
+//
+// Docs/文档:
+//   - https://cppreference.com/w/cpp/language/union.html
+//   - https://github.com/mcpp-community/d2mcpp/blob/main/book/src/cpp11/16-generalized-unions.md
+//
+// 练习交流讨论: http://forum.d2learn.org/category/20
+//
+// Auto-Checker/自动检测命令:
+//
+//   d2x checker generalized-unions
+//
+
+#include <d2x/cpp/common.hpp>
+#include <string>
+#include <new>
+
+// 标签类型 — 标记联合体中哪个成员是活跃的
+enum class Tag {
+    D2X_YOUR_ANSWER, // 整数活跃
+    STRING           // 字符串活跃
+};
+
+// 联合体 — 包含 int 和 std::string, 需手动管理生命周期
+union Data {
+    int i;
+    std::string s;
+
+    Data() : i(0) {}
+    ~Data() {} // 析构由外层根据 tag 决定
+};
+
+// 带标签的鉴别联合体
+struct Value {
+    Tag tag;
+    Data data;
+
+    // 构造 int
+    Value(int val) : tag(D2X_YOUR_ANSWER) {
+        new (&data.i) int(val);
+    }
+
+    // 构造 string
+    Value(const std::string& val) : tag(Tag::STRING) {
+        D2X_YOUR_ANSWER;
+    }
+
+    // 获取 int 成员 — 仅当 tag == INTEGER 时合法
+    int as_int() {
+        return data.i;
+    }
+
+    // 获取 string 成员 — 仅当 tag == STRING 时合法
+    const std::string& as_string() {
+        return data.s;
+    }
+
+    // 析构 — 根据 tag 决定析构哪个成员
+    ~Value() {
+        if (tag == Tag::STRING) {
+            D2X_YOUR_ANSWER;
+        }
+    }
+};
+
+
+int main() {
+
+    // 1. 构造 int 值并验证
+    Value v1(42);
+    d2x_assert(v1.tag == Tag::INTEGER);
+    d2x_assert_eq(v1.as_int(), 42);
+
+    // 2. 构造 string 值并验证
+    Value v2(std::string("hello"));
+    d2x_assert(v2.tag == Tag::STRING);
+    d2x_assert(v2.as_string() == "hello");
+
+    // 3. 从 string 切换到 int
+    {
+        Value v3(std::string("world"));
+        d2x_assert(v3.as_string() == "world");
+
+        // 手动析构 string 成员, 切换到 int
+        v3.data.s.~basic_string();
+        v3.tag = Tag::INTEGER;
+        v3.data.i = 100;
+
+        d2x_assert_eq(v3.as_int(), D2X_YOUR_ANSWER);
+
+        // 离开作用域时 ~Value() 发现 tag == INTEGER, 不析构 string
+    }
+    D2X_WAIT
+
+    return 0;
+}

dslings/cpp11/xmake.lua

@@ -180,6 +180,9 @@ target("cpp11-16-generalized-unions-0")
 target("cpp11-16-generalized-unions-1")
     add_files("16-generalized-unions-1.cpp")
 
+target("cpp11-16-generalized-unions-2")
+    add_files("16-generalized-unions-2.cpp")
+
 -- target: cpp11-17-pod-type
 
 target("cpp11-17-pod-type-0")