Re: Global Dependencies Made Easy

Seungbeom Kim <>
Mon, 2 Apr 2007 00:40:28 CST
Jason Hise wrote:

On Mar 31, 4:32 pm, Seungbeom Kim <> wrote:

I think the article is easy to understand.
Let me give you some minor points.

class logger
     ofstream log;
     logger() : log("log.txt") { }

Why is the constructor private? Since there's no friend, there's no way
to construct this object.

The constructor is private because this class is intended to be a
singleton. Other than the static instance declared in the instance
function, no instances should be able to be created.

Sorry for my silly mistake; I overlooked the whole purpose of the single
pattern. (Gee, wasn't I awake by that time?)

To make up for the mistake and add some new technical C++ content, let
me also suggest using a template class for the singleton pattern:

     template <typename T>
     class singleton
         static T& instance() { static T inst; return inst; }

         singleton() { }
         ~singleton() { }

     private: // not defined
         singleton(const singleton&);
         singleton& operator=(const singleton&);

     class logger : private singleton<logger>
         friend class singleton<logger>;

         using singleton<logger>::instance;
         void log(const char* msg) { ofs << msg << std::endl; }

         logger() : ofs("log.txt") { }
         std::ofstream ofs;

This looks like an overkill for such a simple singleton, but you can add
lots of details (e.g. multi-thread safety) to the singleton template,
which many other classes can reuse without clutters in their own code.

Seungbeom Kim

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