Re: Global Dependencies Made Easy

From:
Seungbeom Kim <musiphil@bawi.org>
Newsgroups:
comp.lang.c++.moderated
Date:
Mon, 2 Apr 2007 00:40:28 CST
Message-ID:
<eupkl1$a04$1@news.Stanford.EDU>
Jason Hise wrote:

On Mar 31, 4:32 pm, Seungbeom Kim <musip...@bawi.org> wrote:

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

class logger
{
private:
     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
     {
     public:
         static T& instance() { static T inst; return inst; }

     protected:
         singleton() { }
         ~singleton() { }

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

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

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

     private:
         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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