使用枚举类的C ++ 11标准一致位掩码 [英] C++11 standard conformant bitmasks using enum class

问题描述

您可以使用enum类实现标准符合（如n3242草案的17.5.2.1.3中所述）类型的安全位掩码吗？按照我的阅读方式，如果类型T支持|，&，^，〜，| =，& =和^ =运算符，则它是位掩码，并且进一步可以执行if（l& r），其中l和r属于T类型。列表中缺少运算符！=和==，并且为了允许排序可能还想重载<。

Can you implement standard conformant (as described in 17.5.2.1.3 of the n3242 draft) type safe bitmasks using enum class? The way I read it, a type T is a bitmask if it supports the |,&,^,~,|=,&= and ^= operators and further you can do if(l&r) where l and r are of type T. Missing from the list are the operator != and == and to allow sorting one probably also wants to overload <.

让运算符起作用只是令人讨厌的样板代码，但我看不到if（l& r）怎么办。至少以下代码不能与GCC一起编译（由于极其危险，因为它允许将错误的隐式转换为int）：

Getting the operators to works is just annoying boilerplate code but I do not see how to do if(l&r). At least the following does not compile with GCC (besides being extremely dangerous as it allows an erroneous implicit conversion to int):

enum class Foo{
    operator bool(){
        return (unsigned)*this;
    }
};

编辑：我现在确定枚举类不能有成员。

I now know for certain that enum classes can not have members. The actual question how to do if(l&r) remains though.

推荐答案

我认为您可以...
您必须添加用于位屏蔽事件的运算符。我在这里没有这样做，但是您可以轻松地重载任何关系运算符。

I think you can... You'll have to add operators for bitmasky things. I didn't do it here but you could easily overload any relational operator.

  /**
   *
   */
  // NOTE: I changed to a more descriptive and consistent name
  //       This needs to be a real bitmask type.
  enum class file_permissions : int
  {
    no_perms        = 0,

    owner_read      =  0400,
    owner_write     =  0200,
    owner_exe       =  0100,
    owner_all       =  0700,

    group_read      =   040,
    group_write     =   020,
    group_exe       =   010,
    group_all       =   070,

    others_read     =    04,
    others_write    =    02,
    others_exe      =    01,
    others_all      =    07,

    all_all     = owner_all | group_all | others_all, // 0777

    set_uid_on_exe  = 04000,
    set_gid_on_exe  = 02000,
    sticky_bit      = 01000,

    perms_mask      = all_all | set_uid_on_exe | set_gid_on_exe | sticky_bit, // 07777

    perms_not_known = 0xffff,

    add_perms       = 0x1000,
    remove_perms    = 0x2000,
    symlink_perms   = 0x4000
  };

  inline constexpr file_permissions
  operator&(file_permissions x, file_permissions y)
  {
    return static_cast<file_permissions>
      (static_cast<int>(x) & static_cast<int>(y));
  }

  inline constexpr file_permissions
  operator|(file_permissions x, file_permissions y)
  {
    return static_cast<file_permissions>
      (static_cast<int>(x) | static_cast<int>(y));
  }

  inline constexpr file_permissions
  operator^(file_permissions x, file_permissions y)
  {
    return static_cast<file_permissions>
      (static_cast<int>(x) ^ static_cast<int>(y));
  }

  inline constexpr file_permissions
  operator~(file_permissions x)
  {
    return static_cast<file_permissions>(~static_cast<int>(x));
  }

  inline file_permissions &
  operator&=(file_permissions & x, file_permissions y)
  {
    x = x & y;
    return x;
  }

  inline file_permissions &
  operator|=(file_permissions & x, file_permissions y)
  {
    x = x | y;
    return x;
  }

  inline file_permissions &
  operator^=(file_permissions & x, file_permissions y)
  {
    x = x ^ y;
    return x;
  }

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