在方法中仅构造一次 std:map

Question

我有这个静态方法：

CString COutlookCalendarSettingsDlg::GetExitCodeAsString(DWORD dwExitCode)
{
    using OutlookExitCodesMap = std::map<DWORD, CString>;

    OutlookExitCodesMap mapExitCodes;

    mapExitCodes.insert(std::pair<DWORD, CString>(1, _T("NoError")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-1, _T("CommandLineArguments")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-2, _T("BuildingCalendarList")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-3, _T("CalendarEventsPathNullEmpty")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-4, _T("CalendarEventsPathNotFound")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-5, _T("ModeSwitchNotSpecified")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-6, _T("ModeSwitchInvalid")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-7, _T("AddEventsMWB")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-8, _T("AddEventsSRR")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-9, _T("SignOut")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-10, _T("ReadMWBData")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-11, _T("ReadSRRData")));

    return mapExitCodes[dwExitCode];
}

现在，我知道我可以将它变成 class 中的全局变量，然后从这个全局变量中获取 GetExitCodeAsString 方法 return。

但是我怎样才能在方法中定义地图，但是，只构造一次呢？我不需要继续重建它。因此，第一次调用它时会构造它，随后的时间它只会 return 值。

这可以做到吗？

Answer 1

答案简单但丑陋：

CString COutlookCalendarSettingsDlg::GetExitCodeAsString(DWORD dwExitCode)
{
  using OutlookExitCodesMap = std::map<DWORD, CString>;

  static OutlookExitCodesMap mapExitCodes;

  if (mapExitCodes.size()==0)
  {
    mapExitCodes.insert(std::pair<DWORD, CString>(1, _T("NoError")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-1, _T("CommandLineArguments")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-2, _T("BuildingCalendarList")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-3, _T("CalendarEventsPathNullEmpty")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-4, _T("CalendarEventsPathNotFound")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-5, _T("ModeSwitchNotSpecified")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-6, _T("ModeSwitchInvalid")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-7, _T("AddEventsMWB")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-8, _T("AddEventsSRR")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-9, _T("SignOut")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-10, _T("ReadMWBData")));
    mapExitCodes.insert(std::pair<DWORD, CString>(-11, _T("ReadSRRData")));
  }
  return mapExitCodes[dwExitCode];
}

这段代码对于多线程来说是不安全的。此外，如果存在未知的退出代码，则地图会增长。不需要这个...

但是为什么要为这么简单的代码使用地图。更简单，不使用任何堆，甚至不使用任何构造：

CString COutlookCalendarSettingsDlg::GetExitCodeAsString(DWORD dwExitCode)
{
  using OutlookExitCodesMap = std::map<DWORD, CString>;


  static const struct { 
    int     dwCode;
    LPCTSTR pszText;
  } 
  aMap[] = 
  {
    1, _T("NoError"),
    -1, _T("CommandLineArguments"),
    -2, _T("BuildingCalendarList"),
    -3, _T("CalendarEventsPathNullEmpty"),
    -4, _T("CalendarEventsPathNotFound"),
    -5, _T("ModeSwitchNotSpecified"),
    -6, _T("ModeSwitchInvalid"),
    -7, _T("AddEventsMWB"),
    -8, _T("AddEventsSRR"),
    -9, _T("SignOut"),
    -10, _T("ReadMWBData"),
    -11, _T("ReadSRRData"),
  };
  for (const auto &data : aMap)
  {
    if (static_cast<DWORD>(data.dwCode)==dwExitCode)
       return data.pszText;
  }

  return CString();
}

代码可能有错别字...我是从头开始写的

Answer 2

这是基于对我的问题提供的评论的另一种解决方案：

CString COutlookCalendarSettingsDlg::GetExitCodeAsString(DWORD dwExitCode)
{
    switch (dwExitCode)
    {
    case 1: return _T("NoError");
    case -1: return _T("CommandLineArguments");
    case -2: return _T("BuildingCalendarList");
    case -3: return _T("CalendarEventsPathNullEmpty");
    case -4: return _T("CalendarEventsPathNotFound");
    case -5: return _T("ModeSwitchNotSpecified");
    case -6: return _T("ModeSwitchInvalid");
    case -7: return _T("AddEventsMWB");
    case -8: return _T("AddEventsSRR");
    case -9: return _T("SignOut");
    case -10: return _T("ReadMWBData");
    case -11: return _T("ReadSRRData");
    }

return CString();

}

在方法中仅构造一次 std:map

Constructing a std:map only once within a method

c++

mfc

stdmap