2.5 处理类型
        类型别名(type alias)       
        typedef  double wages ;
        typedef wages base, *p ; 
  
        或者是using!!别名声明!
              using SI = Sales_item;
        
   2.5.2 auto 类型说明符
        auto itme = val1 + val2 ; 
        
   2.5.3 decltype 类型指示符
        decltype(f()) sum = x;原文:https://www.cnblogs.com/UXCccccc/p/13992095.html