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

extension/some_problem/README.md

@@ -0,0 +1,100 @@
+> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-tools/cpp-string-to-int/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ string 转 int
+
+个人公众号：
+
+![](https://raw.githubusercontent.com/Light-City/CPlusPlusThings/master/img/wechat.jpg)
+
+首先提出解决方案：
+
+- atoi
+- strtol
+- stoi
+
+这几个有什么不同呢？下面测试对比。
+
+> C语言风格函数
+
+atoi与strtol对比：
+
+```cpp
+string str = "16s";
+int a = atoi(str.c_str());
+int b = strtol(str.c_str(), nullptr, 10);
+```
+
+输出：
+
+```
+atoi的结果为:16
+strtol的结果为:16
+```
+
+这两个函数都是从字符串开始寻找数字或者正负号或者小数点,遇到非法字符终止。
+
+所以到上述s字符就不输出了，提前结束，也就是说当你的字符串不是数字的时候，或者小数点等非数字，不会报异常！直接输出0.
+
+例如：
+
+```cpp
+string str = "asdsa";
+int a = atoi(str.c_str());
+int b = strtol(str.c_str(), nullptr, 10);
+```
+
+输出：
+
+```
+0
+0
+```
+
+strtol相比与atoi来说，支持多种进制转换,例如8进制等
+
+例如：
+
+```
+int b = strtol(str.c_str(), nullptr, 8);
+```
+
+> C++风格
+
+在C++中可以使用stoi来转int，这个函数相比于前两个一个最大特点是：异常！
+
+我们知道C++相比于C语言多了异常，这也是这个函数在C++中具有的最显著功能。
+
+例如：
+
+```cpp
+string str1 = "asq,";
+//    int c = stoi(str1);    // 报异常
+string str2 = "12312";
+int c = stoi(str2);     // ok
+cout << c << endl;
+```
+
+异常如下：
+
+```
+terminate called after throwing an instance of 'std::invalid_argument'
+what():  stoi
+```
+
+> 自定义
+
+也就是自己写呗，如下：
+
+```cpp
+int stringToInt(const string &s) {
+    int v;
+    stringstream ss;
+    ss << s;
+    ss >> v;
+    return v;
+}
+int main() {
+    int i = stringToInt("2.3");
+    cout<<i<<endl;
+}
+```