Now lets see our RootHistogramFactory Singleton

class RootHistogramFactory : public HistogramFactory {

public:

// stuff deleted...

static RootHistogramFactory *buildFactory();

static int deleteFactory();

protected:

RootHistogramFactory::RootHistogramFactory();

RootHistogramFactory::~RootHistogramFactory();

private:

static RootHistogramFactory *single;

static int access_count;

};

RootHistogramFactory* RootHistogramFactory::buildFactory()

{

if (single == 0) single = new RootHistogramFactory();

access_count++;

return single;

}

• Nobody can construct an instance of the object because the constructor is protected.

• The only way to get an instance is the static “buildFactory” member function (this function can call the protected constructor)

• it will create at most one instance and subsequent requests will receive a pointer to the only existing instance.

• We keep track of how may copies of the pointer were given out

• Only if the access count is 0 we allow deleting the object again (not shown).

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