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类中是多么的轻易。
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：
空行（认为它是注释中的一部分）
以‘＃’ 开始的注释行
‘<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）。注重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)：假如类中有该属性则返回‘真’
string get_property(strPropertyName)：返回指定的属性（假如指定属性不存在，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（假如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（假如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轻易的一件事。
#include exception>
#include string>
#include sstream>
#include map>
#include vector>
#include fstream>
#include algorithm>
#include functional>
//答应字符串转化
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)'\t';
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 += '\n'; }
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: \n" + 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;
//假如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
int main(int argc, char* argv[])
{
file_reader_writer< char> rw( "properties.txt");
rw.set_property( "App Path", "C:\Program Files\PFS\jokEXPlorer");
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;
}
