book: add memory alignment alignof and alignas

changkun · changkun · commit 115827f98ccc · 2019-07-19T12:00:46.000+02:00
see changkun#2
diff --git a/book/en-us/09-others.md b/book/en-us/09-others.md
@@ -159,6 +159,40 @@ Custom literals support four literals:
 3. String literals: A parameter table of the form `(const char *, size_t)` must be used;
 4. Character literals: Parameters can only be `char`, `wchar_t`, `char16_t`, `char32_t`.
 
+## 9.4 Memory Alignment
+
+C++ 11 introduces two new keywords, `alignof` and `alignas`, to support control of memory alignment.
+The `alignof` keyword can get a platform-dependent value of type `std::size_t` to query the alignment of the platform.
+Of course, we are sometimes not satisfied with this, and even want to customize the alignment of the structure. Similarly, C++ 11 introduces `alignas`.
+To reshape the alignment of a structure. Let's look at two examples:
+
+```cpp
+#include <iostream>
+
+struct Storage {
+    char      a;
+    int       b;
+    double    c;
+    long long d;
+};
+
+struct alignas(std::max_align_t) AlignasStorage {
+    char      a;
+    int       b;
+    double    c;
+    long long d;
+};
+
+int main() {
+    std::cout << alignof(Storage) << std::endl;
+    std::cout << alignof(AlignasStorage) << std::endl;
+    return 0;
+}
+```
+
+where `std::max_align_t` requires exactly the same alignment for each scalar type, so it has almost no difference in maximum scalars.
+In turn, the result on most platforms is `long double`, so the alignment requirement for `AlignasStorage` we get here is 8 or 16.
+
 ## Conclusion
 
 Several of the features introduced in this section are those that 
diff --git a/book/en-us/toc.md b/book/en-us/toc.md
@@ -93,7 +93,7 @@
     + 9.3 Literal
         + Raw String Literal
         + Custom String Literal
-    + 9.4 Math Library
+    + 9.4 Memory Alignment
 - [**Chapter 10 Outlook: Introduction of C++20**](./10-cpp20.md)
     + 10.1 Concept
     + 10.2 Range
diff --git a/book/zh-cn/09-others.md b/book/zh-cn/09-others.md
@@ -145,6 +145,40 @@ int main() {
 3. 字符串字面量：必须使用 `(const char *, size_t)` 形式的参数表；
 4. 字符字面量：参数只能是 `char`, `wchar_t`, `char16_t`, `char32_t` 这几种类型。
 
+## 9.4 内存对齐
+
+C++ 11 引入了两个新的关键字 `alignof` 和 `alignas` 来支持对内存对齐进行控制。
+`alignof` 关键字能够获得一个与平台相关的 `std::size_t` 类型的值，用于查询该平台的对齐方式。
+当然我们有时候并不满足于此，甚至希望自定定义结构的对齐方式，同样，C++ 11 还引入了 `alignas`
+来重新修饰某个结构的对齐方式。我们来看两个例子：
+
+```cpp
+#include <iostream>
+
+struct Storage {
+    char      a;
+    int       b;
+    double    c;
+    long long d;
+};
+
+struct alignas(std::max_align_t) AlignasStorage {
+    char      a;
+    int       b;
+    double    c;
+    long long d;
+};
+
+int main() {
+    std::cout << alignof(Storage) << std::endl;
+    std::cout << alignof(AlignasStorage) << std::endl;
+    return 0;
+}
+```
+
+其中 `std::max_align_t` 要求每个标量类型的对齐方式严格一样，因此它几乎是最大标量没有差异，
+进而大部分平台上得到的结果为 `long double`，因此我们这里得到的 `AlignasStorage` 的对齐要求是 8 或 16。
+
 ## 总结
 
 本节介绍的几个特性是从仍未介绍的现代 C++ 新特性里使用频次较靠前的特性了，`noexcept` 是最为重要的特性，它的一个功能在于能够阻止异常的扩散传播，有效的让编译器最大限度的优化我们的代码。
diff --git a/book/zh-cn/toc.md b/book/zh-cn/toc.md
@@ -93,7 +93,7 @@
     + 9.3 字面量
         + 原始字符串字面量
         + 自定义字面量
-    + 9.4 数学库
+    + 9.4 内存对齐
 - [**第 10 章 展望: C++20 简介**](./10-cpp20.md)
     + 10.1 Concept
     + 10.2 Range
diff --git a/code/9/9.3.alignment.cpp b/code/9/9.3.alignment.cpp
@@ -0,0 +1,21 @@
+#include <iostream>
+
+struct Storage {
+    char      a;
+    int       b;
+    double    c;
+    long long d;
+};
+
+struct alignas(std::max_align_t) AlignasStorage {
+    char      a;
+    int       b;
+    double    c;
+    long long d;
+};
+
+int main() {
+    std::cout << alignof(Storage) << std::endl;
+    std::cout << alignof(AlignasStorage) << std::endl;
+    return 0;
+}
