From 2dae6108f88fc6e9a7a006da5b9d1ac520385cb2 Mon Sep 17 00:00:00 2001
From: li-zhou <2181719471@qq.com>
Date: Thu, 11 Jun 2026 21:50:31 +0800
Subject: [PATCH 1/6] docs(cpp11): add STL real-world case and exercise mapping
 for 16-generalized-unions
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

- 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.
---
 book/src/cpp11/16-generalized-unions.md | 67 +++++++++++++++++++++++--
 1 file changed, 62 insertions(+), 5 deletions(-)

diff --git a/book/src/cpp11/16-generalized-unions.md b/book/src/cpp11/16-generalized-unions.md
index 0c771c7..5fc07f6 100644
--- a/book/src/cpp11/16-generalized-unions.md
+++ b/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_() noexcept {
+        // 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"的语义, 类型安全且更易维护
+
+## 三、注意事项
 
 **可访问性**
 
@@ -170,11 +218,20 @@ 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](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp11/16-generalized-unions-0.cpp) — C++11 规则: 最多一个变体成员可带默认初始化器
+- 1 - [联合体包含非平凡类型及生命周期管理](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp11/16-generalized-unions-1.cpp) — placement new 构造 / 显式析构 / 非平凡成员的正确访问
+
+### 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)

From 99fc7857d9697089e9192a4c119157455f07fb0e Mon Sep 17 00:00:00 2001
From: li-zhou <2181719471@qq.com>
Date: Thu, 11 Jun 2026 22:25:58 +0800
Subject: [PATCH 2/6] docs(cpp11): fix exercise list format to match project
 convention

---
 book/src/cpp11/16-generalized-unions.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/book/src/cpp11/16-generalized-unions.md b/book/src/cpp11/16-generalized-unions.md
index 5fc07f6..bc76368 100644
--- a/book/src/cpp11/16-generalized-unions.md
+++ b/book/src/cpp11/16-generalized-unions.md
@@ -222,8 +222,8 @@ double c = m.b; // 错误：未定义行为
 
 ### 练习代码主题
 
-- 0 - [联合体默认成员初始化](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp11/16-generalized-unions-0.cpp) — C++11 规则: 最多一个变体成员可带默认初始化器
-- 1 - [联合体包含非平凡类型及生命周期管理](https://github.com/mcpp-community/d2mcpp/blob/main/dslings/cpp11/16-generalized-unions-1.cpp) — placement new 构造 / 显式析构 / 非平凡成员的正确访问
+- 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)
 
 ### 练习代码自动检测命令
 

From 359c65309c960d405a383e14a4c47dd9cfe581c0 Mon Sep 17 00:00:00 2001
From: Dragon Roar <2181719471@qq.com>
Date: Fri, 12 Jun 2026 08:41:06 +0800
Subject: [PATCH 3/6] Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
---
 book/src/cpp11/16-generalized-unions.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/book/src/cpp11/16-generalized-unions.md b/book/src/cpp11/16-generalized-unions.md
index bc76368..b6a0db0 100644
--- a/book/src/cpp11/16-generalized-unions.md
+++ b/book/src/cpp11/16-generalized-unions.md
@@ -141,7 +141,7 @@ public:
         _Variant_storage<_Rest...> _Tail;
     };
 
-    _CONSTEXPR20 ~_Variant_storage_() noexcept {
+    _CONSTEXPR20 ~_Variant_storage_() {
         // 联合体不知道哪个成员活跃, 析构由外层 variant 控制
     }
     // ...

From d75b167528a81546ce7fe08bf3a0856b5b9a9166 Mon Sep 17 00:00:00 2001
From: li-zhou <2181719471@qq.com>
Date: Fri, 12 Jun 2026 09:03:41 +0800
Subject: [PATCH 4/6] =?UTF-8?q?docs(cpp11):=20apply=20Copilot=20review=20s?=
 =?UTF-8?q?uggestions=20=E2=80=94=20remove=20inaccurate=20noexcept,=20fix?=
 =?UTF-8?q?=20=E7=B1=BB=E5=9E=8B=E5=AE=89=E5=85=A8=20wording?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 book/src/cpp11/16-generalized-unions.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/book/src/cpp11/16-generalized-unions.md b/book/src/cpp11/16-generalized-unions.md
index b6a0db0..9c725ad 100644
--- a/book/src/cpp11/16-generalized-unions.md
+++ b/book/src/cpp11/16-generalized-unions.md
@@ -172,7 +172,7 @@ class any {
 };
 ```
 
-> 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"的语义, 类型安全且更易维护
+> 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
 

From a378028aef2dad18f70b94382dc72574bbf000f5 Mon Sep 17 00:00:00 2001
From: li-zhou <2181719471@qq.com>
Date: Thu, 11 Jun 2026 23:00:44 +0800
Subject: [PATCH 5/6] feat(cpp11): add tagged-union exercise for
 16-generalized-unions
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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.
---
 book/src/cpp11/16-generalized-unions.md      |   1 +
 dslings/cpp11/16-generalized-unions-2.cpp    | 106 +++++++++++++++++++
 dslings/cpp11/xmake.lua                      |   3 +
 dslings/en/cpp11/16-generalized-unions-2.cpp | 106 +++++++++++++++++++
 dslings/en/cpp11/xmake.lua                   |   3 +
 solutions/cpp11/16-generalized-unions-2.cpp  |  82 ++++++++++++++
 solutions/cpp11/xmake.lua                    |   3 +
 7 files changed, 304 insertions(+)
 create mode 100644 dslings/cpp11/16-generalized-unions-2.cpp
 create mode 100644 dslings/en/cpp11/16-generalized-unions-2.cpp
 create mode 100644 solutions/cpp11/16-generalized-unions-2.cpp

diff --git a/book/src/cpp11/16-generalized-unions.md b/book/src/cpp11/16-generalized-unions.md
index 9c725ad..3934262 100644
--- a/book/src/cpp11/16-generalized-unions.md
+++ b/book/src/cpp11/16-generalized-unions.md
@@ -224,6 +224,7 @@ double c = m.b; // 错误：未定义行为
 
 - 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
 
diff --git a/dslings/cpp11/16-generalized-unions-2.cpp b/dslings/cpp11/16-generalized-unions-2.cpp
new file mode 100644
index 0000000..906285e
--- /dev/null
+++ b/dslings/cpp11/16-generalized-unions-2.cpp
@@ -0,0 +1,106 @@
+// 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;
+}
diff --git a/dslings/cpp11/xmake.lua b/dslings/cpp11/xmake.lua
index 3f7d247..e7dbdae 100644
--- a/dslings/cpp11/xmake.lua
+++ b/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")
diff --git a/dslings/en/cpp11/16-generalized-unions-2.cpp b/dslings/en/cpp11/16-generalized-unions-2.cpp
new file mode 100644
index 0000000..2759e37
--- /dev/null
+++ b/dslings/en/cpp11/16-generalized-unions-2.cpp
@@ -0,0 +1,106 @@
+// d2mcpp: https://github.com/mcpp-community/d2mcpp
+// license: Apache-2.0
+// file: dslings/en/cpp11/16-generalized-unions-2.cpp
+//
+// Exercise: cpp11 | 16 - generalized unions | tagged discriminated union
+//
+// Tips:
+// - Use an enum to tag which member of the union is active
+// - Choose constructor/destructor based on the tag
+// - The union itself cannot track which member is active; the tag is maintained externally
+//
+// Docs:
+//   - https://cppreference.com/w/cpp/language/union.html
+//   - https://github.com/mcpp-community/d2mcpp/blob/main/book/en/src/cpp11/16-generalized-unions.md
+//
+// Discussion Forum: http://forum.d2learn.org/category/20
+//
+// Auto-Checker:
+//
+//   d2x checker generalized-unions
+//
+
+#include <d2x/cpp/common.hpp>
+#include <string>
+#include <new>
+
+// Tag type — marks which member of the union is active
+enum class Tag {
+    D2X_YOUR_ANSWER,// integer is active
+    STRING           // string is active
+};
+
+// Union — contains int and std::string, lifecycle managed externally
+union Data {
+    int i;
+    std::string s;
+
+    Data() : i(0) {}
+    ~Data() {} // destruction decided by outer struct based on tag
+};
+
+// Tagged discriminated union
+struct Value {
+    Tag tag;
+    Data data;
+
+    // Construct with int
+    Value(int val) : tag(D2X_YOUR_ANSWER) {
+        new (&data.i) int(val);
+    }
+
+    // Construct with string
+    Value(const std::string& val) : tag(Tag::STRING) {
+        D2X_YOUR_ANSWER;
+    }
+
+    // Access int member — valid only when tag == INTEGER
+    int as_int() {
+        return data.i;
+    }
+
+    // Access string member — valid only when tag == STRING
+    const std::string& as_string() {
+        return data.s;
+    }
+
+    // Destructor — destroy the active member based on tag
+    ~Value() {
+        if (tag == Tag::STRING) {
+            D2X_YOUR_ANSWER;
+        }
+    }
+};
+
+
+int main() {
+
+    // 1. Construct an int value and verify
+    Value v1(42);
+    d2x_assert(v1.tag == Tag::INTEGER);
+    d2x_assert_eq(v1.as_int(), 42);
+
+    // 2. Construct a string value and verify
+    Value v2(std::string("hello"));
+    d2x_assert(v2.tag == Tag::STRING);
+    d2x_assert(v2.as_string() == "hello");
+
+    // 3. Switch from string to int
+    {
+        Value v3(std::string("world"));
+        d2x_assert(v3.as_string() == "world");
+
+        // Manually destroy the string member, then switch to int
+        v3.data.s.~basic_string();
+        v3.tag = Tag::INTEGER;
+        v3.data.i = 100;
+
+        d2x_assert_eq(v3.as_int(), D2X_YOUR_ANSWER);
+
+        // When v3 goes out of scope, ~Value() sees tag == INTEGER, skips string destruction
+    }
+
+    D2X_WAIT
+
+    return 0;
+}
diff --git a/dslings/en/cpp11/xmake.lua b/dslings/en/cpp11/xmake.lua
index 3f60434..6daeb9e 100644
--- a/dslings/en/cpp11/xmake.lua
+++ b/dslings/en/cpp11/xmake.lua
@@ -177,6 +177,9 @@ 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")
diff --git a/solutions/cpp11/16-generalized-unions-2.cpp b/solutions/cpp11/16-generalized-unions-2.cpp
new file mode 100644
index 0000000..4f4a067
--- /dev/null
+++ b/solutions/cpp11/16-generalized-unions-2.cpp
@@ -0,0 +1,82 @@
+// d2mcpp: https://github.com/mcpp-community/d2mcpp
+// license: Apache-2.0
+// reference solution for: dslings/cpp11/16-generalized-unions-2.cpp
+//
+// 用途: 仅给 CI 与维护者参考使用，不是教程入口。
+// 教程练习入口: dslings/cpp11/16-generalized-unions-2.cpp
+//
+
+#include <d2x/cpp/common.hpp>
+#include <string>
+#include <new>
+
+enum class Tag {
+    INTEGER,
+    STRING
+};
+
+union Data {
+    int i;
+    std::string s;
+
+    Data() : i(0) {}
+    ~Data() {}
+};
+
+struct Value {
+    Tag tag;
+    Data data;
+
+    Value(int val) : tag(Tag::INTEGER) {
+        new (&data.i) int(val);
+    }
+
+    Value(const std::string& val) : tag(Tag::STRING) {
+        new (&data.s) std::string(val);
+    }
+
+    int as_int() {
+        return data.i;
+    }
+
+    const std::string& as_string() {
+        return data.s;
+    }
+
+    ~Value() {
+        if (tag == Tag::STRING) {
+            data.s.~basic_string();
+        }
+    }
+};
+
+
+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(), 100);
+
+        // 离开作用域时 ~Value() 发现 tag == INTEGER, 不析构 string
+    }
+
+    return 0;
+}
diff --git a/solutions/cpp11/xmake.lua b/solutions/cpp11/xmake.lua
index 6cfcfce..31cdd51 100644
--- a/solutions/cpp11/xmake.lua
+++ b/solutions/cpp11/xmake.lua
@@ -183,6 +183,9 @@ target("cpp11-16-generalized-unions-0-ref")
 target("cpp11-16-generalized-unions-1-ref")
     add_files("16-generalized-unions-1.cpp")
 
+target("cpp11-16-generalized-unions-2-ref")
+    add_files("16-generalized-unions-2.cpp")
+
 -- target: cpp11-17-pod-type
 
 target("cpp11-17-pod-type-0-ref")

From eef9661daeec08b6d5815832b540de5386ac3f08 Mon Sep 17 00:00:00 2001
From: li-zhou <2181719471@qq.com>
Date: Sat, 13 Jun 2026 19:43:00 +0800
Subject: [PATCH 6/6] 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
---
 book/en/src/cpp11/16-generalized-unions.md   | 69 ++++++++++++++++++--
 dslings/cpp11/16-generalized-unions-2.cpp    |  1 -
 dslings/en/cpp11/16-generalized-unions-2.cpp |  3 +-
 3 files changed, 65 insertions(+), 8 deletions(-)

diff --git a/book/en/src/cpp11/16-generalized-unions.md b/book/en/src/cpp11/16-generalized-unions.md
index 27fb0a2..26ef95c 100644
--- a/book/en/src/cpp11/16-generalized-unions.md
+++ b/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)
diff --git a/dslings/cpp11/16-generalized-unions-2.cpp b/dslings/cpp11/16-generalized-unions-2.cpp
index 906285e..57bd336 100644
--- a/dslings/cpp11/16-generalized-unions-2.cpp
+++ b/dslings/cpp11/16-generalized-unions-2.cpp
@@ -99,7 +99,6 @@ int main() {
 
         // 离开作用域时 ~Value() 发现 tag == INTEGER, 不析构 string
     }
-
     D2X_WAIT
 
     return 0;
diff --git a/dslings/en/cpp11/16-generalized-unions-2.cpp b/dslings/en/cpp11/16-generalized-unions-2.cpp
index 2759e37..5cd28dc 100644
--- a/dslings/en/cpp11/16-generalized-unions-2.cpp
+++ b/dslings/en/cpp11/16-generalized-unions-2.cpp
@@ -98,8 +98,7 @@ int main() {
         d2x_assert_eq(v3.as_int(), D2X_YOUR_ANSWER);
 
         // When v3 goes out of scope, ~Value() sees tag == INTEGER, skips string destruction
-    }
-
+    }    
     D2X_WAIT
 
     return 0;