Dr. Dobb's Journal - October 2007 - (Page 53)

const int notifiedClients = NotifyClients(); LOG(logINFO) << “A number of “ << notifiedClients << “ were notified.”; Going Generic Another issue with the implementation we built so far is that the code is hardwired to log to stderr, only stderr, and nothing but stderr. If your library is part of a GUI application, it would make more sense to be able to log to an ASCII file. The client (not the library) should specify what the log destination is. It’s not difficult to parameterize Log to allow changing the destination FILE*, but why give Log a fish when we could teach it fishing? A better approach is to completely separate our Log-specific logic from the details of lowlevel output. The communication can be done in an efficient manner through a policy class. Using policy-based design is justified (in lieu of a more dynamic approach through runtime polymorphism) by the argument that, unlike logging level, it’s more likely you decide the logging strategy upfront, at design time. So let’s change Log to define and use a policy. Actually, the policy interface is very simple as it models a simple string sink: static void Output(const std::string& msg); The Log class morphs into a class template that expects a policy implementation: template class Log { // }; template Log ::~Log() { OutputPolicy::Output(msg.str()); } That’s pretty much all that needs to be done on Log. You can now provide the FILE* output simply as an implementation of the OutputPolicy policy; see Listing Two. The code below shows how Listing One // Log, version 0.1: a simple logging class enum TLogLevel {logERROR, logWARNING, logINFO, logDEBUG, logDEBUG1, logDEBUG2, logDEBUG3, logDEBUG4}; class Log { public: Log(); virtual ~Log(); std::ostringstream& Get(TLogLevel level = logINFO); public: static TLogLevel& ReportingLevel(); protected: std::ostringstream os; private: Log(const Log&); Log& operator =(const Log&); private: TLogLevel messageLevel; }; std::ostringstream& Log::Get(TLogLevel level) { os << "- " << NowTime(); os << " " << ToString(level) << ": "; os < logDEBUG ? 0 : level - logDEBUG, '\t'); messageLevel = level; return os; } Log::~Log() { if (messageLevel >= Log::ReportingLevel()) { os << std::endl; fprintf(stderr, "%s", os.str().c_str()); fflush(stderr); } } October 2007 l www.ddj.com l Dr. Dobb’s Journal 53 http://www.faircom.com/go/?useDLD http://www.faircom.com/go/?useDLD http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - October 2007

Cover Contents Hmmmm Alia Vox Developer Diaries Developer’s Notebook AI: It’s OK Again! Conversations Visual Cryptography and Bit-Plane Complexity Segmentation Inside the Windows Vista Disk Encryption Algorithm Memory-Aware Components Software and the Core Description Process Logging In C++ Effective Concurrency The Agile Edge Swaine’s Flames

Dr. Dobb's Journal - October 2007

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