编译器扩展与标准对齐控制

https://github.com/huihut/interview/issues/110

README.md

@@ -290,24 +290,43 @@ assert( p != NULL );    // assert 不可用
 * sizeof 对数组，得到整个数组所占空间大小。
 * sizeof 对指针，得到指针本身所占空间大小。
 
-### #pragma pack(n)
+### 编译器扩展与标准对齐控制
 
-设定结构体、联合以及类成员变量以 n 字节方式对齐
+* 编译器扩展：`#pragma pack(n)`，将随后定义的 `struct`/`class`/`union` 的成员最大对齐限制为 n 字节。
+* 标准对齐控制：
+    * `alignas(k)`，要求类型或变量至少按 k 字节对齐（向上取整到 ≥ 自然对齐）。
+    * `alignof(T)`，获取类型 T 的自然对齐要求（编译时常量）。
 
-#pragma pack(n) 使用
+特性    |#pragma pack|alignas
+--------|--------------|------------
+标准化  |❌ 编译器扩展  |   ✅ C++11标准
+对齐方向|⬇️ 只能减小对齐|    ⬆️ 只能增大对齐
+可移植性|❌ 编译器依赖  |   ✅ 跨平台
+作用范围|🔄 影响整个结构|   🎯 可针对单个成员
+性能影响|⚠️ 可能降低内存访问速度|   ⚠️ 过度对齐浪费空间
+
+使用
 
 ```cpp
-#pragma pack(push)  // 保存对齐状态
-#pragma pack(4)     // 设定为 4 字节对齐
+#include <cstddef>
+#include <iostream>
 
-struct test
-{
-    char m1;
-    double m4;
-    int m3;
+#pragma pack(push, 1)             // 最大对齐 1 字节，紧凑布局
+struct PackedHeader {
+    uint16_t len;                // offset 0
+    uint32_t id;                 // offset 2
+};
+#pragma pack(pop)
+
+struct alignas(8) Align8 {
+    double  value;               // offset 0, 占 8 字节
+    int     flag;                // offset 8
 };
 
-#pragma pack(pop)   // 恢复对齐状态
+int main() {
+    std::cout << "PackedHeader size: " << sizeof(PackedHeader) << "\n"; // 6
+    std::cout << "Align8 size: " << sizeof(Align8) << "\n";             // 16
+}
 ```
 
 ### 位域

README_en.md

@@ -295,25 +295,46 @@ assert( p != NULL );    // assert is not available
 * sizeof For arrays - get the size of the entire array.
 * sizeof For pointers - get the size of the space occupied by the pointer itself.
 
-### #pragma pack(n)
+### Compiler Extensions vs Standard Alignment Control
 
-Set structure, union, and class member variables to be n-byte aligned
+* Compiler Extension `#pragma pack(n)`, restricts the maximum alignment of members in subsequently defined `struct`/`class`/`union` to n bytes.
+* Standard Alignment Control:
+    * `alignas(k)`, requires types or variables to be aligned to at least k bytes (rounds up to ≥ natural alignment).
+    * `alignof(T)`, gets the natural alignment requirement of type T (compile-time constant).
 
-#pragma pack (n) use
+Feature         | `#pragma pack`         | `alignas` 
+----------------|-------------------------|---------------------
+Standardization | ❌ Compiler Extension  | ✅ C++11 Standard
+Alignment Direction | ⬇️ Only decreases alignment | ⬆️ Only increases alignment
+Portability     | ❌ Compiler Dependent   | ✅ Cross-platform
+Scope           | 🔄 Affects entire struct | 🎯 Per-member control
+Performance Impact | ⚠️ May reduce memory access speed | ⚠️ Over-alignment wastes space
+
+#### Usage Examples
 
 ```cpp
-#pragma pack(push)  // save alignment state
-#pragma pack(4)     // Set to 4 byte alignment
+#include <cstddef>
+#include <iostream>
 
-struct test
-{
-    char m1;
-    double m4;
-    int m3;
+#pragma pack(push, 1)             // Max alignment 1 byte (compact layout)
+struct PackedHeader {
+    uint16_t len;                // offset 0
+    uint32_t id;                 // offset 2
+};
+#pragma pack(pop)
+
+struct alignas(8) Align8 {        // Force 8-byte alignment
+    double  value;               // offset 0 (8 bytes)
+    int     flag;                // offset 8
 };
 
-#pragma pack(pop)   // Restore alignment
-```
+int main() {
+    std::cout << "PackedHeader size: " 
+              << sizeof(PackedHeader) << "\n";  // Output: 6
+    
+    std::cout << "Align8 size: " 
+              << sizeof(Align8) << "\n";        // Output: 16
+}
 
 ### Bit field