使用basic_message _handler_log建立日志类

开发一个日志类比较困难，因为std::basic_stream(buf)内部机理很复杂。这一文章演示了如何通过basic_message_handler_log类很方便地建立一个日志类。

使用std::basic_ostream类来建立自己的日志类，这一方法没有提供任何的控制符号(如<<)。即使想把一些控制符号加载到类中，比如，如果你想把类的对象传递到std::basic_ostream的函数，只有基类的控制符号可以使用。唯一可以控制的是当程序流溢出的时候。这一方法的优点是它效率比较高。如果程序流溢出，则使用std::endl，如下所示：

std::cout << "UserName " << strUserName; // this will write ' ' and flush the stream std::cout << std::endl;

• 通过basic_message_handler_log类生成.
• 调用on_new_message函数

这里提供了使用message_handler_log和 wmessage_handler_log的范例。

#include <time.h> #include <iostream> #include <message_handler_log.h> class CPrefixByTimeLog : public message_handler_log { public: CPrefixByTimeLog( std::ostream & streamOut) : m_streamOut( streamOut) {} void on_new_message( const string_type & strNewMessage) { write_time(); m_streamOut << strNewMessage; m_streamOut.flush(); } private: void write_time() { time_t timeNow = time( NULL); tm timeDetails = *gmtime( &timeNow); std::string strTime = asctime( & timeDetails); strTime.erase( strTime.end() - 1); // forget the ending ' ' m_streamOut << strTime << ": "; } private: // the stream we write to std::ostream & m_streamOut; }; int main(int argc, char* argv[]) { CPrefixByTimeLog time_cout( std::cout); time_cout << "Hello World!" << std::endl; std::string strUserName = "john", strPassword( "****"); time_cout << "New login for user name = " << strUserName << ", Password = " << strPassword << std::endl; return 0; } Here's the code for basic_message_handler_log class: // message_handler_log.h #include <ostream> #include <sstream> #include <memory> #include <assert.h> // forward declaration(s) template< class char_type, class traits_type = std::char_traits< char_type> > class basic_message_handler_log; // represents the stream buffer for a message_handler_log (see below) // Note: NOT thread-safe template< class char_type , class traits_type = std::char_traits< char_type> > class basic_message_handler_log_streambuf : public std::basic_streambuf< char_type, traits_type> { private: friend basic_message_handler_log< char_type, traits_type>; typedef std::basic_streambuf< char_type, traits_type> streambuf_type; typedef basic_message_handler_log< char_type, traits_type> ostream_type; // base class typedef std::basic_streambuf< char_type, traits_type> base_class; enum { _BADOFF = -1 /* bad offset - for positioning functions */ }; protected: // input, not allowed virtual int_type pbackfail(int_type = traits_type::eof()) { // only for output, not for input assert( false); return (traits_type::eof()); } virtual int showmanyc() { // only for output, not for input assert( false); return 0; } virtual int_type underflow() { // only for output, not for input assert( false); return (traits_type::eof()); } virtual int_type uflow() { // only for output, not for input assert( false); return (traits_type::eof()); } virtual std::streamsize xsgetn(char_type * _S, std::streamsize _N) { // only for output, not for input assert( false); return 0; } // positioning, not allowed - we're a log virtual pos_type seekoff(off_type, std::ios_base::seekdir, std::ios_base::openmode = std::ios_base::in | std::ios_base::out) { // we don't allow positioning assert( false); return (std::streampos( _BADOFF)); } virtual pos_type seekpos(pos_type, std::ios_base::openmode = std::ios_base::in | std::ios_base::out) { // we don't allow positioning assert( false); return (std::streampos( _BADOFF)); } // output functions // called to write out from the internal // buffer, into the external buffer virtual int sync() { m_pOwnerStream->on_new_message( get_stream_buffer().str() ); m_pStreamBuffer = std::auto_ptr< string_stream_type>( new string_stream_type); return 0; } virtual streambuf_type *setbuf( char_type * buffer, std::streamsize n) { // ... note: this function MUST be called // before working with this stream buffer // we don't use a buffer - we forward everything assert( buffer == NULL && n == 0); setp( NULL, NULL); return this; } // write the characters from the buffer // to their real destination virtual int_type overflow(int_type nChar = traits_type::eof()) { if ( traits_type::not_eof( nChar)) get_stream_buffer() << ( char_type)nChar; return traits_type::not_eof( nChar); } virtual std::streamsize xsputn(const char_type *S, std::streamsize N) { get_stream_buffer().write( S, N); return N; } public: basic_message_handler_log_streambuf() : m_pStreamBuffer( new string_stream_type) {} private: typedef std::basic_ostringstream< char_type> string_stream_type; string_stream_type & get_stream_buffer() { return *m_pStreamBuffer; } private: // holds the Message, until it's flushed std::auto_ptr< string_stream_type> m_pStreamBuffer; // the Message Handler Log - where we write into ostream_type * m_pOwnerStream; }; // derive your class from this, and implement the PROTECTED on_new_message function template< class char_type, class traits_type = std::char_traits< char_type> > class basic_message_handler_log : public std::basic_ostream< char_type, traits_type> { typedef basic_message_handler_log_streambuf< char_type, traits_type> handler_streambuf_type; friend handler_streambuf_type; typedef std::basic_ostream< char_type, traits_type> base_class; protected: typedef std::basic_string< char_type> string_type; basic_message_handler_log() : m_StreamBuf(), base_class( NULL) { m_StreamBuf.m_pOwnerStream = this; init( & m_StreamBuf); m_StreamBuf.pubsetbuf( NULL, 0); } virtual ~basic_message_handler_log() {} protected: virtual void on_new_message( const string_type & strNewMessage) = 0; public: // our stream buffer handler_streambuf_type m_StreamBuf; }; typedef basic_message_handler_log< char> message_handler_log; typedef basic_message_handler_log< wchar_t> wmessage_handler_log;