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

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- 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
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basic_content/this/README.md
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@@ -1,10 +1,12 @@
# this指针那些事
> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-things/this/) 同步生成，网站版本含多语言与更新排版。
# C++ this 指针
## 关于作者
微信公众号：
![](../img/wechat.jpg)
![](https://raw.githubusercontent.com/Light-City/CPlusPlusThings/master/img/wechat.jpg)
## 1.this指针
@@ -74,16 +76,16 @@ int main(){
为了验证this指针是哪一个，现在在`add_age`处添加断点，运行后如下：
![thiscontrust](./img/thiscontrust.png)
![thiscontrust](https://raw.githubusercontent.com/Light-City/CPlusPlusThings/master/basic_content/this/img/thiscontrust.png)
![genthis](./img/genthis.png)
![genthis](https://raw.githubusercontent.com/Light-City/CPlusPlusThings/master/basic_content/this/img/genthis.png)
会发现编译器自动为我们加上`A* const`，而不是`A const *this`！
紧接着，上述还有个常函数，那么我们在对`get_age`添加断点，如下：
![constthis](./img/constthis.png)
![constthis](https://raw.githubusercontent.com/Light-City/CPlusPlusThings/master/basic_content/this/img/constthis.png)
会发现编译器把上述的this，变为`const A* const`，这个大家也能想到，因为这个函数是const函数，那么针对const函数，它只能访问const变量与const函数，不能修改其他变量的值，所以需要一个this指向不能修改的变量，那就是`const A*`,又由于本身this是`const`指针，所以就为`const A* const`!
总结：this在成员函数的开始执行前构造，在成员的执行结束后清除。上述的get_age函数会被解析成`get_age(const A * const this)`,`add_age`函数会被解析成`add_age(A* const this,int a)`。在C++中类和结构是只有一个区别的：类的成员默认是private，而结构是public。this是类的指针，如果换成结构，那this就是结构的指针了。
总结：this在成员函数的开始执行前构造，在成员的执行结束后清除。上述的get_age函数会被解析成`get_age(const A * const this)`,`add_age`函数会被解析成`add_age(A* const this,int a)`。在C++中类和结构是只有一个区别的：类的成员默认是private，而结构是public。this是类的指针，如果换成结构，那this就是结构的指针了。