什么是表驱动

当if...else等条件调用很多时，将其设计成表驱动的方式可简化代码和提高可维护性。表驱动简单讲（我的理解）就是将条件和约束置入表格中，然后通过表操作来统一执行条件转移操作。如下代码存在这个问题：

5 太多的if-else而不会用表驱动
ppro/PrivacyProtection/rule/KSystemEnvirVar.h

class KSystemEnvirVar
{
public:
// MichaelPeng: 应为静态函数
CString GetValue(LPCTSTR szVariable)
{
TCHAR szFolderPath[MAX_PATH +
1] = {0};
// MichaelPeng: if else太多,应做成表驱动

if (0
== _tcsicmp(szVariable, _T("%Desktop%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_DESKTOP, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Internet%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_INTERNET, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Programs%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_PROGRAMS, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Controls%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_CONTROLS, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Printers%"))) {
http://hi.baidu.com/lewutian
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_PRINTERS, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Personal%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_PERSONAL, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Favorites%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_FAVORITES, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Startup%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_STARTUP, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Recent%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_RECENT, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Sendto%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_SENDTO, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Bitbucket%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_BITBUCKET, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%StartMenu%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_STARTMENU, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Mydocuments%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_MYDOCUMENTS, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Mymusic%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_MYMUSIC, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Myvideo%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_MYVIDEO, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Desktopdirectory%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_DESKTOPDIRECTORY, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Drives%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_DRIVES, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Network%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_NETWORK, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Nethood%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_NETHOOD, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Fonts%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_FONTS, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Templates%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_TEMPLATES, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%CommonStartMenu%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_COMMON_STARTMENU, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%CommonPrograms%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_COMMON_PROGRAMS, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%CommonStartup%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_COMMON_STARTUP, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%LocalAppdate%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_LOCAL_APPDATA, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%CommonAppdata%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_COMMON_APPDATA, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%Windows%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_WINDOWS, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%System32%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_SYSTEM, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%ProgramFilesComm%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_PROGRAM_FILES_COMMON, 0);
} else
if (0
== _tcsicmp(szVariable, _T("%CommonDocuments%"))) {
::SHGetSpecialFolderPath(NULL, szFolderPath, CSIDL_COMMON_DOCUMENTS, 0);
}
else
{
CString strVariable(szVariable);
strVariable.Remove(_T('%'));

TCHAR* szTemp = _tgetenv(strVariable);
if (szTemp)
{
if (_tcsstr(szTemp, L"~"))
{
::GetLongPathNameW(szTemp, szFolderPath, MAX_PATH);
}
else
{
_tcsncpy(szFolderPath, szTemp, MAX_PATH);
}
szFolderPath[MAX_PATH] =
0;
}
}

return szFolderPath;
}

protected:

};

更加详细的说明可以参考这里：

http://baike.baidu.com/view/3707325.htm