C++有符号和无符号之间的转换问题

#include<iOStream>
 
int main()
{
    unsigned a=5;
    int b=-10;
    std::cout<<b+b<<std::endl;//正常输出
    std::cout<<a+b<<std::endl;
 
    return 0;
}

#include<iostream>
 
int main()
{
      unsigned a=10,b=5;
      std::cout<<b-a<<std::endl;
 
      return 0;
}

#include<iostream>
 
int main()
{
     unsigned u=5;
     while(u>=0)
     {
          --u;
          std::cout<<u<<std::endl;
     }
 
     return 0;
 
}