limit_num_of_object.cpp

// only a single object allowed
struct PrintJob
{
    PrintJob(const string& name) : Name(name)
    {
    }
    string Name;
};

class Printer
{
    public:
        static Printer& Instance();
        void SubmitJob(const PrintJob& job) const;

    private:
        Printer()
        {}
        Printer(const Printer& other);
};

Printer& Printer::Instance() 
{
    static Printer printer;
    return printer;
}

void Printer::SubmitJob(const PrintJob& job) const
{
    cout<<"submiting job now..."<<endl;
    cout<<"job name: "<<job.Name<<endl;
    cout<<"submitted"<<endl;
}


// alow a specific number of objects
class Printer
{
    public:
        ~Printer();
        static Printer* Instance();
        static Printer* Instance(const Printer& other);
        void SubmitJob(const PrintJob& job) const;

    private:
        Printer();
        Printer(const Printer& other);

    private:
        static size_t _numObj;
        enum {MAX_NUM_OF_OBJ_ALLOWED = 2};
};


Printer::Printer()
{
    ++_numObj;
}

Printer::Printer(const Printer& other)
{
    ++_numObj;
}

Printer::~Printer()
{
    --_numObj;
}

Printer* Printer::Instance() 
{
    if (_numObj >= MAX_NUM_OF_OBJ_ALLOWED)
    {
        return nullptr;
    }

    return new Printer;
}

Printer* Printer::Instance(const Printer& other)
{
    if (_numObj >= MAX_NUM_OF_OBJ_ALLOWED)
    {
        return nullptr;
    }
    
    return new Printer(other);
}

void Printer::SubmitJob(const PrintJob& job) const
{
    cout<<"submiting job now..."<<endl;
    cout<<"job name: "<<job.Name<<endl;
    cout<<"submitted"<<endl;
}

int main()
{
    //Printer::Instance().SubmitJob(PrintJob("CLRS.txt"));
    unique_ptr<Printer> p1(Printer::Instance());   
    unique_ptr<Printer> p2(Printer::Instance());
    p1->SubmitJob(PrintJob("CLRS"));
    p2->SubmitJob(PrintJob("TAOCP"));
    Printer* p3 = Printer::Instance();
    if (p3 == nullptr)
    {
        cout<<"reach the maximum number of objects"<<endl;
    }
    return 0;
}