C++ singleton pattern question

ferdinand.stefanus

Hi

Could someone tell me what's the difference between these two singleton
implementations :

First implementation:

class Singleton
{
public:
static Singleton& instance() { return _instance; }
//... other members
private:
Singleton();
static Singleton _instance;
//... other members
};

Second implementation:

class Singleton
{
public:
static Singleton* instance() { return _instance; }
static void Initialize() { _instance = new Singleton; }
//... other members
private:
Singleton();
static Singleton* _instance;
//... other members
};

I know that in the first implementation, _instance will be put in the
static memory, and it will always be present, whereas in the second
implementation, memory will only be allocated on the heap after
Singleton::Init ialize() is called. But aside from that, is there any
practical difference between the two, like on what circumstances is the
first implementation preferred over the second (or vice versa)?

Thanks!

Jul 23 '05 #1

Subscribe Reply

5310

adbarnet

For one thing, the pointer implementation has no clean up code - so any
resources the singleton uses may not be returned.

regards,

Aiden
<fe************ ****@gmail.com> wrote in message
news:11******** **************@ l41g2000cwc.goo glegroups.com.. .

Hi

Could someone tell me what's the difference between these two singleton
implementations :

First implementation:

class Singleton
{
public:
static Singleton& instance() { return _instance; }
//... other members
private:
Singleton();
static Singleton _instance;
//... other members
};

Second implementation:

class Singleton
{
public:
static Singleton* instance() { return _instance; }
static void Initialize() { _instance = new Singleton; }
//... other members
private:
Singleton();
static Singleton* _instance;
//... other members
};

I know that in the first implementation, _instance will be put in the
static memory, and it will always be present, whereas in the second
implementation, memory will only be allocated on the heap after
Singleton::Init ialize() is called. But aside from that, is there any
practical difference between the two, like on what circumstances is the
first implementation preferred over the second (or vice versa)?

Thanks!

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Jul 23 '05 #2

ferdinand.stefanus

Yes, but let's say I add another method, say CleanUp(), in the second
implementation that deletes _instance, and users are required to call
Initialize and CleanUp before and after using the class, respectively.
Are the first and second now functionally identical? Or is there a
situation where one implementation is better than the other? (e.g. in a
multi-threaded application, etc.)?

Jul 23 '05 #3

adbarnet

Apart from the explicit vs implicit memory management there is no real
difference - as far as multi - threading is concerned, you need to employ
some synchronisation primitives to protect simulataneous access to your
singleton - the same situation exists in both cases.

The 'popular' issue with threading, is a third possibile implementation
whereby:
MySingleton * instance()
{
static MySingleton * pInst = new MySingleton();

return pInst;
}

That changes your synchronisation requirements - but how is very dependent
on your particular design/usage.

hope that helps,

regards,
Aiden

<fe************ ****@gmail.com> wrote in message
news:11******** *************@o 13g2000cwo.goog legroups.com...

Yes, but let's say I add another method, say CleanUp(), in the second
implementation that deletes _instance, and users are required to call
Initialize and CleanUp before and after using the class, respectively.
Are the first and second now functionally identical? Or is there a
situation where one implementation is better than the other? (e.g. in a
multi-threaded application, etc.)?

Jul 23 '05 #4

adbarnet

Yes, but let's say I add another method, say CleanUp(), in the second
implementation that deletes _instance, and users are required to call
Initialize and CleanUp before and after using the class, respectively.
Are the first and second now functionally identical? Or is there a
situation where one implementation is better than the other? (e.g. in a
multi-threaded application, etc.)?

Jul 23 '05 #5

adbarnet

Yes, but let's say I add another method, say CleanUp(), in the second
implementation that deletes _instance, and users are required to call
Initialize and CleanUp before and after using the class, respectively.
Are the first and second now functionally identical? Or is there a
situation where one implementation is better than the other? (e.g. in a
multi-threaded application, etc.)?

Jul 23 '05 #6

adbarnet

Yes, but let's say I add another method, say CleanUp(), in the second
implementation that deletes _instance, and users are required to call
Initialize and CleanUp before and after using the class, respectively.
Are the first and second now functionally identical? Or is there a
situation where one implementation is better than the other? (e.g. in a
multi-threaded application, etc.)?

Jul 23 '05 #7

Spacen Jasset

adbarnet wrote:

Apart from the explicit vs implicit memory management there is no real
difference - as far as multi - threading is concerned, you need to employ
some synchronisation primitives to protect simulataneous access to your
singleton - the same situation exists in both cases.

The 'popular' issue with threading, is a third possibile implementation
whereby:
MySingleton * instance()
{
static MySingleton * pInst = new MySingleton();

return pInst;
}

....

Doing is this way, with a static 'inside' a function guarantees that
MySingleton will be initialised even if you try and access it before
main() is called. e.g from another singleton, or initialisation.

Jul 23 '05 #8

Tomas

Ok, just to be complete, lets add the fourth implementation

class Singelton
{
Singelton();

public:

static Singelton& Instance() { static Singelton s; return s; }
};

cheers

Jul 23 '05 #9

Rolf Magnus

fe************* ***@gmail.com wrote:

Yes, but let's say I add another method, say CleanUp(), in the second
implementation that deletes _instance, and users are required to call
Initialize and CleanUp before and after using the class, respectively.
Are the first and second now functionally identical? Or is there a
situation where one implementation is better than the other? (e.g. in a
multi-threaded application, etc.)?

Read up on the "static initialization order fiasco" in this newsgroup's FAQ.

Jul 23 '05 #10

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