Re: Instance() in Singleton Class Question

From:

Vladimir Jovic <vladaspams@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 13 Apr 2009 07:53:30 +0200

Message-ID:

<gruk0q$fv$1@news01.versatel.de>

Immortal Nephi wrote:

I am curious to ask. Why do you need to use Instance() function?
You can always use Singleton() function and ~Singleton() function.

The constructor and destructor are protected, therefore no access to them

Execute Instance() function will start to execute Singleton() function
before it returns back to Instance() function. The program exited
before it will not execute ~Singleton() function. You must use
Terminate() function manually to clear dynamic memory otherwise leak
memory can occur. It is not worth practice.

Not really. Normal OS will clean up after finished process.

Read Note 1 and Note 2 inside main() function. Note 2 does its own
job to execute ~Singleton() function before program exits. Note 2 is
always useful when you want to use only one instance (one object such
as keyboard, mouse, floppy drive, etc)...

#include <stdio.h>
#include <stdlib.h>

class Singleton
{
public:
static Singleton* Instance(); // Why do you need it?

To get the pointer to the only instance of this object.

void Terminate(); // Why do you need it?

This method is not needed

     static void Print();

//protected:
    Singleton();
    ~Singleton();

private:
    Singleton(const Singleton&);
    Singleton& operator= (const Singleton&);

    static Singleton* pinstance; // Why do you need it?
};

Singleton* Singleton::pinstance = 0;

Singleton* Singleton::Instance ()
{
    if (pinstance == 0)
        pinstance = new Singleton;

    return pinstance;
}

void Singleton::Terminate ()
{
    if (pinstance != 0)
    {
        delete pinstance;
        pinstance = 0;
    }
}

Singleton::Singleton()
{
    printf("Constructor\n");
}

Singleton::~Singleton()
{
    printf("Destructor\n");
}

void Singleton::Print()
{
    printf("Test\n");
}

int main()
{
    // Note 1
    Singleton & ref = * Singleton::Instance();
    ref.Print();
    ref.Terminate();

    // Note 2
    Singleton s;

Don't you get an error? The constructor is protected

     s.Print(); // ( Singleton::Print() is same as s.Print() )

    system("pause"); // Function comes from Microsoft Visual C++ .Net

    return 0;
}