C++ 作用域内枚举

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——传统的枚举存在一些问题,其中之一是两个枚举类型定义中的枚举量可能发生冲突。

enum egg {Small, Medium, Large, Jumbo};
enum t_shirt {Small, Medium, Large, Xlarge};

  这将无法通过编译(egg Small和t_shirt Small发生冲突)

解决办法(新枚举):

C++11提供了一种新枚举,其枚举量的作用域为类:

enum class egg {Small, Medium, Large, Jumbo};
eum class t_shirt {Small, Medium, Large, Xlarge};
  • 可使用关键字struct代替class
  • 使用时需使用枚举名来限定枚举量:
    egg choice = egg::Large;    // the Large enumerator of the egg enum
    t_shirt Floyd = t_shirt::Large;    // the Large enumerator of the t_shirt enum
    
  • C++11提高了作用域内枚举的类型安全:
    • 有时,常规枚举将自动转换为整型,如将其赋给int变量或用于比较表达式时,但作用域内枚举不能隐式地转换为整型:
      enum egg_old {Small, Medium, Large, Jumbo};    // unscoped
      enum class t_shirt {Small, Medium, Large, Xlarge};    // scoped
      egg_old one = Medium;    // unscoped
      t_shirt rolf = t_shirt::Large;    // scoped
      int king = one;    // implicit type conversion ofr unscoped
      int ring = rolf;    // not allowed, no implicit type conversion
      if (king < Jumbo)    // allowed
          std::cout<<"Jumbo converted to int before comparison.\n";
      if (king < t_shirt::Medium)    // not allowed
          std::cout<<"Not allowed: < not defined for scoped enum.\n";
      

        但在必要时,可显示类型转换:

      int Frodo = int(t_shirt::Small);    // Frodo set to 0
      

        

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