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-cpp11-variadic-templates-cpp.md

@@ -0,0 +1,146 @@
+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/modern-cpp/cpp11/variadic-templates-cpp/) 的文章，由站点同步脚本导出。
+
+# C++ 可变参数模板
+
+## 《C++那些事》之可变参数模版
+
+最近在看Arrow的源代码发现了一个比较有意思的设计，在一个plan中可以添加多个节点，例如：proj、agg等等。
+
+今天来讲讲这一块的设计与实现。
+
+## 1.添加节点
+
+对于一个plan通常需要添加一堆节点，这里便是调用EmplaceNode来实现的。
+
+```cpp
+return plan->EmplaceNode<ScalarAggregateNode>(
+    plan, std::move(inputs), schema(std::move(fields)), std::move(target_field_ids),
+    std::move(aggregates), std::move(kernels), std::move(states),
+    std::move(owned_options));
+```
+
+同理，如果添加一个proj节点就是:
+
+```cpp
+return plan->EmplaceNode<ProjectNode>(plan, std::move(inputs),
+                schema(std::move(fields)), std::move(exprs),
+                project_options.async_mode);
+```
+
+那么究竟是什么魅力让代码写的如此丝滑？
+
+这里把Plan简单写一下：
+
+```cpp
+class ARROW_EXPORT ExecPlan
+{
+	public:
+    template <typename Node, typename... Args>
+      Node* EmplaceNode(Args&&... args) {
+        std::unique_ptr<Node> node{new Node{std::forward<Args>(args)...}};
+        auto out = node.get();
+        AddNode(std::move(node));
+        return out;
+      }
+ }
+```
+
+看上这个语法非常的复杂，没错，它是11之后的可变参数模版，这里解释一下这段代码的含义，就是将参数包丢给对应的Node构造函数。这个例子可能看起来比较复杂，接下来我们进入Demo环节。
+
+### 2.Demo
+
+接下来我们创建了一个图，图中有两个不同的节点，分别是节点 A与 节点B，为了简单器件这里使用一个函数模版EmplaceNode。
+
+```cpp
+NodeA* nodeA = EmplaceNode<NodeA>("Node A", 10, 3.14);
+NodeB* nodeB = EmplaceNode<NodeB>(1, "Node B");
+```
+
+完整代码如下：
+
+```cpp
+#include <iostream>
+#include <vector>
+#include <memory>
+
+class NodeA {
+public:
+    NodeA(const std::string& name, int value, double weight)
+        : name_(name), value_(value), weight_(weight) {}
+
+    const std::string& GetName() const {
+        return name_;
+    }
+
+private:
+    std::string name_;
+    int value_;
+    double weight_;
+};
+
+class NodeB {
+public:
+    NodeB(int id, const std::string& desc)
+        : id_(id), desc_(desc) {}
+
+    int GetID() const {
+        return id_;
+    }
+
+private:
+    int id_;
+    std::string desc_;
+};
+template<typename Node, typename... Args>
+Node* EmplaceNode(Args&&... args) {
+    std::unique_ptr<Node> node{new Node{std::forward<Args>(args)...}};
+    auto out = node.get();
+    // AddNode(std::move(node));
+    return out;
+}
+
+int main() {
+    NodeA* nodeA = EmplaceNode<NodeA>("Node A", 10, 3.14);
+    NodeB* nodeB = EmplaceNode<NodeB>(1, "Node B");
+    std::cout << "Node A name: " << nodeA->GetName() << std::endl;
+    std::cout << "Node B ID: " << nodeB->GetID() << std::endl;
+    return 0;
+}
+```
+
+## 3.拓展-实现一个print函数可以打印任意类型数据
+
+简单实现版本：
+
+使用可变模版参数+递归方式实现即可。
+
+每次拆分成一个+一堆，直到只剩下一个，所以有一个处理一个的函数与处理一个+一堆的函数，分别对应第一个print_impl与第二个print_impl。
+
+```cpp
+#include <iostream>
+#include <utility>
+
+template <typename T>
+void print_impl(T&& t) {
+    std::cout << t;
+}
+
+template <typename T, typename... Args>
+void print_impl(T&& t, Args&&... args) {
+    std::cout << t;
+    print_impl(std::forward<Args>(args)...);
+}
+
+template <typename... Args>
+void print(Args&&... args) {
+    print_impl(std::forward<Args>(args)...);
+}
+
+int main() {
+    int x = 1;
+    const char* str = "Hello, world!";
+    print("The value of x is ", x, ", and the string is ", str, ".\n");
+    return 0;
+}
+
+```