book: add C++17 aligned new for over-aligned types

Extend 9.4 with dynamic allocation of over-aligned types: C++17 added
operator new/delete overloads taking std::align_val_t, so new respects
alignof for over-aligned types. Compiled example code/9/9.4. Refs #1

Author: changkun

book/en-us/09-others.md

@@ -191,6 +191,20 @@ int main() {
 where `std::max_align_t` requires 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.
 
+### Dynamic allocation of over-aligned types
+
+Before C++17, a `new` expression could not guarantee the alignment requirement of an **over-aligned** type (one whose alignment exceeds `alignof(std::max_align_t)`); using such types often required platform-specific facilities such as `posix_memalign` or `_aligned_malloc`. C++17 introduced `operator new` / `operator delete` overloads taking a `std::align_val_t`, so a `new` expression automatically selects the aligned version when allocating an over-aligned type:
+
+```cpp
+struct alignas(64) Aligned {
+    double v[8];
+};
+
+Aligned* p = new Aligned; // C++17: automatically uses the aligned operator new
+// now reinterpret_cast<std::uintptr_t>(p) % 64 == 0
+delete p;
+```
+
 ## Conclusion
 
 Several of the features introduced in this section are those that

book/zh-cn/09-others.md

@@ -179,6 +179,20 @@ int main() {
 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。
 
+### 过对齐类型的动态分配
+
+在 C++17 之前，`new` 表达式无法保证满足**过对齐 (over-aligned)** 类型（即对齐要求超过 `alignof(std::max_align_t)` 的类型）的对齐需求，使用这类类型常常需要借助平台特定的 `posix_memalign`、`_aligned_malloc` 等手段。C++17 引入了接受 `std::align_val_t` 参数的 `operator new` / `operator delete` 重载，使得 `new` 表达式在为过对齐类型分配内存时会自动选用对齐版本：
+
+```cpp
+struct alignas(64) Aligned {
+    double v[8];
+};
+
+Aligned* p = new Aligned; // C++17：自动使用对齐版本的 operator new
+// 此时 reinterpret_cast<std::uintptr_t>(p) % 64 == 0
+delete p;
+```
+
 ## 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。

code/9/9.4.aligned.new.cpp

@@ -0,0 +1,28 @@
+//
+// 9.4.aligned.new.cpp
+// chapter 09 others
+// modern c++ tutorial
+//
+// created by changkun at changkun.de
+// https://github.com/changkun/modern-cpp-tutorial
+//
+
+#include <cstdint>
+#include <iostream>
+#include <new>
+
+// Over-aligned type: its alignment exceeds the default new alignment.
+struct alignas(64) Aligned {
+    double v[8];
+};
+
+int main() {
+    // C++17: new automatically uses the aligned operator new for
+    // over-aligned types, so the result respects alignof(Aligned).
+    Aligned* p = new Aligned;
+    std::cout << "alignof  = " << alignof(Aligned) << "\n";
+    std::cout << "aligned? = "
+              << (reinterpret_cast<std::uintptr_t>(p) % alignof(Aligned) == 0)
+              << "\n";
+    delete p;
+}