很方便地将对象读出/写入文件流

当处理对象操作时，程序员通常会从文件流中读出或写入对象。在一般情况下，你使用>>或<<操作符号。然而，这一方法很繁琐，并且你必需调用一定的函数来访问这一对象的数据。

一种很方便的对象读写操作方法是调用read_from_stream和 write_to_stream函数。这是很好的设计思路，这一方法可以很容易地执行或避免函数的调用。

调用read_from_stream 和write_to_stream函数的方法有以下两种：

1.从stream_friendly< your_class_name>中获取你的类

2.从virtual_stream_friendly< your_class_name, char_type>中获取你的类

在你的类中会自动地定义操作符>>和<<，即在内部上自动调用read_from_stream和 write_to_stream。

如何在这两种方法中选择其中一种？以下是选择的依据：

这里是stream_friendly and virtual_stream_friendly类使用方法的范例。

#include <iosfwd> template< class T> class stream_friendly { /* // this is how your functions should look like template< class CharType, class CharTraits> void write_to_stream( std::basic_ostream< CharType, CharTraits> & streamOut) const { // code } template< class CharType, class CharTraits> void read_from_stream( std::basic_istream< CharType, CharTraits> & streamIn) { // code } */ }; template< class CharType, class CharTraits, class T> std::basic_ostream< CharType, CharTraits> & operator<< ( std::basic_ostream< CharType, CharTraits> & streamOut, const stream_friendly< T> & object) { (( const T &)object).write_to_stream( streamOut); return streamOut; } template< class CharType, class CharTraits, class T> std::basic_istream< CharType, CharTraits> & operator>> ( std::basic_istream< CharType, CharTraits> & streamIn, stream_friendly< T> & object) { (( T &)object).read_from_stream( streamIn); return streamIn; } template< class T, class CharType, class CharTraits = std::char_traits< CharType> > class virtual_stream_friendly { public: typedef std::basic_ostream< CharType, CharTraits> ostream_type; typedef std::basic_istream< CharType, CharTraits> istream_type; virtual void write_to_stream( ostream_type & streamOut) const {} virtual void read_from_stream( ostream_type & streamIn) {} }; template< class CharType, class CharTraits, class T> std::basic_ostream< CharType, CharTraits> & operator<< ( std::basic_ostream< CharType, CharTraits> & streamOut, const virtual_stream_friendly< T, CharType, CharTraits> & object) { object.write_to_stream( streamOut); return streamOut; } template< class CharType, class CharTraits, class T> std::basic_istream< CharType, CharTraits> & operator>> ( std::basic_istream< CharType, CharTraits> & streamIn, virtual_stream_friendly< T, CharType, CharTraits> & object) { object.read_from_stream( streamIn); return streamIn; }

下面是每个类的例子:

#include <string> #include <iostream> #include <vector> #include <iterator> ///////////////////////////////////////////////// // Testing stream_friendly class class Name : public stream_friendly< Name> { public: Name() {} void write_to_stream( std::ostream & streamOut) const { streamOut << m_strFirstName << " " << m_strLastName; } void read_from_stream( std::istream & streamIn) { streamIn >> m_strFirstName >> m_strLastName; } private: std::string m_strFirstName; std::string m_strLastName; }; void Test_stream_friendly() { // Testing stream_friendly class Name your_name; std::cout << "Enter your name (two words):" << std::endl; std::cin >> your_name; std::cout << "Your name is " << your_name << std::endl; } ///////////////////////////////////////////////// // Testing virtual_stream_friendly class struct Window : public virtual_stream_friendly< Window, char> { long m_hWnd; std::string m_strWindowName; virtual void write_to_stream( ostream_type & streamOut) const { streamOut << "hwnd: " << m_hWnd << ", Window name: '" << m_strWindowName << "' "; } }; struct ListBox : public Window { virtual void write_to_stream( ostream_type & streamOut) const { Window::write_to_stream( streamOut); streamOut << " Window items: "; std::copy( m_astrItems.begin(), m_astrItems.end(), std::ostream_iterator< std::string>( streamOut, ", ")); } std::vector< std::string> m_astrItems; }; void Test_virtual_stream_friendly() { Window wnd; wnd.m_hWnd = 0x552235; wnd.m_strWindowName = "test base window"; std::cout << wnd << std::endl ; ListBox box; box.m_hWnd = 0x8842200; box.m_strWindowName = "test list box"; box.m_astrItems.push_back( "first item"); box.m_astrItems.push_back( "second item"); box.m_astrItems.push_back( "last item"); std::cout << box << std::endl; } int main(int argc, char* argv[]) { Test_stream_friendly(); Test_virtual_stream_friendly(); return 0; }