feat: optimize vol2 passages, in ch05 and ch00 (#115)

Author: Charliechen114514

code/examples/vol2/13_init_statements.cpp

@@ -0,0 +1,67 @@
+// 13_init_statements.cpp
+// if/switch 初始化器：把变量的生存范围卡在它真正有用的那几行
+
+#include <iostream>
+#include <map>
+#include <mutex>
+#include <string>
+
+struct Command {
+    int type;
+    int arg;
+};
+
+Command read_command(int raw) {
+    return {raw / 10, raw % 10};
+}
+
+int main() {
+    std::cout << "=== if/switch 初始化器演示 ===\n\n";
+
+    std::map<int, std::string> cache = {{1, "one"}, {2, "two"}};
+
+    // 1. map 查找：迭代器被关在 if/else 内，出了块就不可见
+    std::cout << "1. map 查找 (if init):\n";
+    if (auto it = cache.find(2); it != cache.end()) {
+        std::cout << "  找到: " << it->second << "\n";
+    } else {
+        std::cout << "  没找到\n";
+    }
+    std::cout << "\n";
+
+    // 2. insert + 结构化绑定：声明、判断、使用一行搞定
+    std::cout << "2. insert + 结构化绑定:\n";
+    if (auto [it, ok] = cache.insert({3, "three"}); ok) {
+        std::cout << "  插入成功: " << it->second << "\n";
+    } else {
+        std::cout << "  已存在: " << it->second << "\n";
+    }
+    std::cout << "\n";
+
+    // 3. 锁守卫：锁的持有范围精确匹配条件块（CTAD 省去模板参数）
+    std::cout << "3. 锁守卫:\n";
+    std::mutex mtx;
+    bool ready = true;
+    if (std::lock_guard lock(mtx); ready) {
+        std::cout << "  持锁状态下执行\n";
+    }
+    std::cout << "\n";
+
+    // 4. switch 初始化器：init 里准备数据，switch 里分发
+    std::cout << "4. switch 初始化器:\n";
+    for (int raw : {12, 25, 31}) {
+        switch (auto cmd = read_command(raw); cmd.type) {
+            case 1:
+                std::cout << "  cmd " << raw << " -> start(" << cmd.arg << ")\n";
+                break;
+            case 2:
+                std::cout << "  cmd " << raw << " -> stop(" << cmd.arg << ")\n";
+                break;
+            default:
+                std::cout << "  cmd " << raw << " -> unknown\n";
+                break;
+        }
+    }
+
+    return 0;
+}

documents/en/vol2-modern-features/ch00-move-semantics/03-rvo-nrvo.md

@@ -392,12 +392,6 @@ Run the structured-binding examples online and see unpacking for `pair`, `tuple`
   allow-run
 />
 
-## Wrapping up
-
-That's the full coverage of types structured binding handles: `pair`, `tuple`, native arrays, structs with public members, plus custom types that implement the tuple-like protocol. The semantics are entirely decided by the modifier in front of `auto`—`auto` copies, `auto&` references, `const auto&` is read-only, `auto&&` forwards.
-
-What I actually use day to day is range-based for over a map (`for (const auto& [k, v] : m)`) and catching multi-return functions. Pair it with the if/switch initializers in the next chapter and your code shrinks another size.
-
 ## References
 
 - [cppreference: Structured binding declaration](https://en.cppreference.com/w/cpp/language/structured_binding)