使用Properties类带来的好处（第一部分）

许多开发者抱怨C＋＋不能像Java那样绑定Properties类。Java的Properties类内在包含一个文件，该文件用来读写Properties类中的属性，可以写成这样形式：<名字>＝<数值>（例如：ConnectToInternet＝Use IE）。

使用Properties类的好处就是你可以很轻松的理解和修改它们。在本文的第一部分中，你将看到我们也可以在C＋＋中使用Properties类。本文的第二部分将向你演示通过使用操作符>>和<<把数据保存到Properties类中是多么的容易。

现在介绍C＋＋ Properties文件的结构。该文件的每一行可以是下面三种情况中的某一种：

1. 空行（认为它是注释中的一部分）
2. 以‘＃’ 开始的注释行
3. ‘<名字>＝<数值>’行，这是给一个属性赋值的语句

现在让我们再看看Properties类的的特点：

• 注释是持久性的（当保存Properties类时，它们不会丢失掉）。注意每一个注释都属于某个属性。在‘<名字>＝<数值>’行上的注释行属于该‘<名字>’属性。
• 当保存Properties类后，属性仍然保留自己的位置。
• 它对各种字符类型都有效：char、wchar_t等等

Properties类的使用相当简单：

• save()：保存属性
• has_property(strPropertyName)：如果类中有该属性则返回‘真’
• string get_property(strPropertyName)：返回指定的属性（如果指定属性不存在，则抛出例外）
• set_property(strPropertyName, strPropertyValue)：设置给定属性
• stringget_property_comment( strPropertyName)：返回属于指定属性的注释（如果指定属性的注释不存在，则抛出例外）
• set_property_comment(strPropertyName, strPropertyComment)：设置指定属性的注释（如果指定属性的注释不存在，则抛出例外）

下面是file_reader_writer类以及相应例子的代码。运行它之后，请查看properties.txt文件。看看访问和修改它是多么容易的一件事。

#include exception> #include string> #include sstream> #include map> #include vector> #include fstream> #include algorithm> #include functional> //允许字符串转化 template< class FromCharType, class ToCharType> inline std::basic_string< ToCharType> convert_string( const std::basic_string< FromCharType> & strSource) { std::basic_string< ToCharType> strDest; int nSourceLen = strSource.length(); strDest.resize( nSourceLen); for ( int idxChar = 0; idxChar < nSourceLen; idxChar++) { strDest[ idxChar] = ( ToCharType)strSource[ idxChar]; } return strDest; } // 用于trim_spaces; template< class CharType> struct is_char_in_str : public std::binary_function< CharType, std::basic_string< CharType>, bool> { bool operator()( CharType ch, const std::basic_string< CharType> & strSource) const { return (strSource.find( ch) != std::basic_string< CharType>::npos); } }; //消除字符串中的空格 template< class CharType> std::basic_string< CharType> trim_spaces( const std::basic_string< CharType> & strSource) { std::basic_string< CharType> strSpaces; strSpaces += ( CharType)' '; strSpaces += ( CharType)' '; typedef std::basic_string< CharType> string_type; string_type::const_iterator itFirst = std::find_if( strSource.begin(), strSource.end(), std::not1( std::bind2nd( is_char_in_str< CharType>(), strSpaces))); string_type::const_reverse_iterator ritLast = std::find_if( strSource.rbegin(), strSource.rend(), std::not1( std::bind2nd( is_char_in_str< CharType>(), strSpaces))); string_type::const_iterator itLast = &*ritLast; if ( itFirst <= itLast) if ( itFirst != strSource.end()) return string_type( itFirst, itLast + 1); return string_type(); } // 当读写属性时的例外 class properties_exception : public std::exception { public: properties_exception( const std::string & str) : m_strDescription( str) {} const char * what() const { return m_strDescription.c_str(); } private: std::string m_strDescription; }; // 从文件中读写属性 template< class CharType> class file_reader_writer { typedef std::basic_string< CharType> string_type; public: // ... needed within the basic_properties! typedef CharType char_type; public: file_reader_writer( const char * strFileName) : m_bIsDirty( false), m_strFileName( strFileName) { read_properties(); } ~file_reader_writer() { save(); } void save() { write_properties(); } bool has_property( const string_type & strPropertyName) const { PropertiesCollection::const_iterator itFound = m_collProperties.find( strPropertyName); return ( itFound != m_collProperties.end()); } const string_type & get_property( const string_type & strPropertyName) const { PropertiesCollection::const_iterator itFound = m_collProperties.find( strPropertyName); if ( itFound != m_collProperties.end()) return itFound->second.m_strValue; else throw properties_exception( "Cound not get property value for '" + convert_string< char_type, char>( strPropertyName) + "', since this property does not exist."); } void set_property( const string_type & strProperty, const string_type & strPropertyValue) { PropertiesCollection::iterator itFound = m_collProperties.find( strProperty); if ( itFound == m_collProperties.end()) // 它是一个新的属性 m_aProperties.push_back( strProperty); m_collProperties[ strProperty].m_strValue = strPropertyValue; m_bIsDirty = true; } const string_type & get_property_comment( const string_type & strPropertyName) const { PropertiesCollection::const_iterator itFound = m_collProperties.find( strPropertyName); if ( itFound != m_collProperties.end()) return itFound->second.m_strComment; else throw properties_exception( "Cound not get property comment for '" + convert_string< char_type, char>( strPropertyName) + "', since this property does not exist."); } void set_property_comment( const string_type & strPropertyName, const string_type & strPropertyComment) { PropertiesCollection::iterator itFound = m_collProperties.find( strPropertyName); if ( itFound != m_collProperties.end()) itFound->second.m_strComment = strPropertyComment; else throw properties_exception( "Cound not set property comment for '" + convert_string< char_type, char>( strPropertyName) + "', since this property does not exist."); m_bIsDirty = true; } private: static const char_type get_delimeter() { return '='; } static const char_type get_comment() { return '#'; } void read_properties() { const char DELIMETER = get_delimeter(); const char COMMENT = get_comment(); std::basic_ifstream< char_type> streamIn( m_strFileName.c_str()); string_type strLine; string_type strComment; while ( std::getline( streamIn, strLine)) { strLine = trim_spaces( strLine); bool bIsComment = strLine.empty() || ( strLine[ 0] == COMMENT); if ( bIsComment) { strComment += strLine; strComment += ' '; } else { int idxDelimeter = strLine.find( DELIMETER); if ( idxDelimeter != string_type::npos) { string_type strPropertyName = strLine.substr( 0, idxDelimeter); string_type strPropertyValue = strLine.substr( idxDelimeter + 1); strPropertyName = trim_spaces( strPropertyName); strPropertyValue = trim_spaces( strPropertyValue); m_collProperties.insert( std::make_pair( strPropertyName, OneProperty( strPropertyValue, strComment))); m_aProperties.push_back( strPropertyName); strComment.erase(); } else throw properties_exception( "While reading from file '" + m_strFileName + "', we encountered an invalid line: " + convert_string< char_type, char>( strLine)); } } m_strLastComment = strComment; } void write_properties() const { if ( !m_bIsDirty) // 无需保存 ;return; const char DELIMETER = get_delimeter(); std::basic_ofstream< char_type> streamOut( m_strFileName.c_str()); PropertiesArray::const_iterator itFirst = m_aProperties.begin(), itLast = m_aProperties.end(); while ( itFirst != itLast) { const string_type & strPropertyName = *itFirst; const OneProperty & property = m_collProperties.find( strPropertyName)->second; write_property_comment( streamOut, property.m_strComment); string_type strToWrite = strPropertyName; strToWrite += ' '; strToWrite += DELIMETER; strToWrite += ' '; streamOut strToWrite << property.m_strValue << std::endl; ++itFirst; } write_property_comment( streamOut, m_strLastComment); m_bIsDirty = false; } void write_property_comment( std::basic_ofstream< char_type> & streamOut, const string_type & strComment) const { const char COMMENT = get_comment(); std::basic_stringstream< char_type> streamComment( strComment); string_type strLine; while ( std::getline( streamComment, strLine)) { if ( !strLine.empty()) if ( strLine[ 0] == COMMENT) streamOut << strLine << std::endl; else { string_type strPrefix; strPrefix += COMMENT; strPrefix += ' '; streamOut << strPrefix << strLine << std::endl; } else streamOut << std::endl; } } private: // 我们用来读写的文件 std::string m_strFileName; //如果自上次保存后属性又有修改则赋值为“真” mutable bool m_bIsDirty; struct OneProperty { OneProperty() {} OneProperty( const string_type & strValue, const string_type & strComment) : m_strValue( strValue), m_strComment( strComment) {} string_type m_strValue; string_type m_strComment; }; // 属性 typedef std::map< string_type, OneProperty> PropertiesCollection; PropertiesCollection m_collProperties; // ……确保我们是按同样的次序保存属性 // 读属性 typedef std::vector< string_type> PropertiesArray; PropertiesArray m_aProperties; // 读取所有属性后的注释 string_type m_strLastComment; };

下面是用到这个类的一个例子：

#include <iostream> int main(int argc, char* argv[]) { file_reader_writer< char> rw( "properties.txt"); rw.set_property( "App Path", "C:Program FilesPFSjokExplorer"); rw.set_property_comment( "App Path", "where are we installed?"); rw.set_property( "Version", "4.0.0.1"); rw.set_property_comment( "Version", "What's our version?"); rw.set_property( "Run On Startup", "1"); rw.set_property_comment( "Run On Startup", "are we run, when the computer starts?"); rw.set_property( "Automatic Logoff Minutes", "60"); rw.set_property_comment( "Automatic Logoff Minutes", "when should we deconnect from the server?"); rw.set_property( "Connect To Internet", "Use IE"); rw.set_property_comment( "Connect To Internet", "how are we to connect to the Internet?"); std::cout << "This is how we connect to Internet: " << rw.get_property( "Connect To Internet") << std::endl; return 0; }