docs: address community issue feedback (#5 #338 #142 #246 #136 #303)

- const README: rewrite the space-saving claim per issue #5 follow-ups
  (constant-expression const vs #define immediates; which const qualify
  as constant expressions)
- const README: fix the class-section diagnosis per issue #338 (overload
  resolution picks the const overload; the restriction only surfaces
  once that overload is removed)
- sizeof README: virtual-inheritance result is compiler-dependent
  (GCC merges vbptr+vfptr -> 16, MSVC keeps them separate -> 32) #246
- static README: new section on internal linkage / access restriction
  with anonymous-namespace comparison #142
- 10-day practice: full problem list table + fixed day links #136
- new design_pattern/singleton/singleton_default.hpp: reusable
  thread-safe CRTP singleton template, compile+concurrency tested #303

site_extras/modern-cpp-cpp17-structured-bindings.md

@@ -0,0 +1,117 @@
+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/modern-cpp/cpp17/structured-bindings/) 的文章，由站点同步脚本导出。
+
+# C++17 结构化绑定
+
+在聊结构化绑定之前，有几个面试问题，看看你会不会？
+
+- 如何使用结构化绑定访问自定义类的私有成员？
+
+- 如何使用结构化绑定修改自定义类的成员呢？
+
+这几个题目估计没几个人能答上来，题目与答案网上也都没有，基于这些问题，我们引出本文的内容：如何为自定义类型支持结构化绑定，里面有哪些坑？
+
+**注：懒人版，答案与代码已放星球。**
+
+为了让本文的内容更加丝滑，从示例出发。
+
+## 1.自定义类型
+
+如果给定一个类：
+
+```cpp
+struct Foo {
+  Foo(int64_t id, std::string data) : id_(id), data_(std::move(data)) {}
+  void Print() { std::cout << "id = " << id_ << ", data = " << data_ << std::endl; }
+
+ private:
+  int64_t id_;
+  std::string data_;
+};
+```
+
+要让下面这个代码运行起来：
+
+```cpp
+int main() {
+  Foo f{12, "hello world"};
+  const auto& [id, data] = f;
+  std::cout << "id = " << id << ", data = " << data << std::endl;
+  f.Print();
+}
+```
+
+如果直接运行会编译：
+
+```cpp
+ cannot decompose inaccessible member 'Foo::id_' of 'const Foo'
+   42 |   const auto& [id, data] = f;
+```
+
+那么怎么做呢？
+
+## 2.支持结构化绑定
+
+这个问题我们使用两个步骤：
+
+1.特化 `std::tuple_size` 和 `std::tuple_element`
+
+```cpp
+namespace std {
+  template <>
+  struct tuple_size<Foo> : std::integral_constant<std::size_t, 2> {};
+
+  template <>
+  struct tuple_element<0, Foo> {
+    using type = int64_t;
+  };
+
+  template <>
+  struct tuple_element<1, Foo> {
+    using type = std::string;
+  };
+}
+```
+
+2.为上面的结构体添加友元get函数，然后实现即可：
+
+```cpp
+template <std::size_t Index>
+decltype(auto) get(const Foo& f);
+
+template <>
+decltype(auto) get<0>(const Foo& f) {
+  return f.id_;
+}
+
+template <>
+decltype(auto) get<1>(const Foo& f) {
+  return f.data_;
+}
+```
+
+此时便可以正常的工作了，得到输出：
+
+```cpp
+id = 12, data = hello world
+id = 12, data = hello world
+```
+
+最后，抛出几个问题：
+
+- 上面提供的是只读访问，如果在访问的时候修改const auto &会发生什么？
+
+例如：
+
+```cpp
+auto& [id, data] = f;
+```
+
+- 如何支持对类成员的修改访问？
+
+即，让这个语句能够正常运行
+
+```cpp
+auto& [id, data] = f;
+```
+
+好了，本节完，欢迎留言区交流。