How to change instance variable value in instance function

[Ed]

Hi, guys,
I am wondering if I could change the static variable value in one
static function?

If I have a class:

class Apple
{
public:
static Apple* Instance()
{
static Apple* pInstance(NULL) ;
if( ! pInstance)
{
pInstance = new Apple();
}
return pInstance;
}
}


Can I change the pInstance from outside?
Something like: Apple::Instance ()::pInstance = NULL;

Thanks!

[Jeroen Mostert]

Re: How to change instance variable value in instance function

Ed wrote:

If I have a class:
>
class Apple
{
public:
static Apple* Instance()
{
static Apple* pInstance(NULL) ;
if( ! pInstance)
{
pInstance = new Apple();
}
return pInstance;
}
}
>
>
Can I change the pInstance from outside?

No, unless you somehow pass a pointer or reference to the variable to the
outside world from the function. But that's such a mind-bogglingly silly
violation of encapsulation that it's not worth considering.

If you want this ability, then don't use function-scoped static variables.
Use a static class member instead. If you can't modify the original class,
then consider the possibility that there's a very good reason why you're not
supposed to change this value from the outside.

--
J.

[David Wilkinson]

Re: How to change instance variable value in instance function

Jeroen Mostert wrote:

Ed wrote:

>If I have a class:
>>
>class Apple
>{
>public:
> static Apple* Instance()
> {
> static Apple* pInstance(NULL) ;
> if( ! pInstance)
> {
> pInstance = new Apple();
> }
> return pInstance;
> }
>}
>>
>>
>Can I change the pInstance from outside?

>
No, unless you somehow pass a pointer or reference to the variable to
the outside world from the function. But that's such a mind-bogglingly
silly violation of encapsulation that it's not worth considering.
>
If you want this ability, then don't use function-scoped static
variables. Use a static class member instead. If you can't modify the
original class, then consider the possibility that there's a very good
reason why you're not supposed to change this value from the outside.

Ed:

I don't know why you want to do this, but one way would be

static Apple* Instance(bool bReset = false)
{
static Apple* pInstance(NULL) ;
if( bReset )
{
delete pInstance;
pInstance = NULL;
}
else if( ! pInstance )
{
pInstance = new Apple();
}
return pInstance;
}

BTW, since this is a question on standard C++ (not C++/CLI), you would be better
to ask it in microsoft.publi c.vc.language.

--
David Wilkinson
Visual C++ MVP

[Ed]

Re: How to change instance variable value in instance function

On Aug 3, 9:58 pm, David Wilkinson <no-re...@effisols. comwrote:

Jeroen Mostert wrote:

Ed wrote:

If I have a class:

>

class Apple
{
public:
static Apple* Instance()
{
static Apple* pInstance(NULL) ;
if( ! pInstance)
{
pInstance = new Apple();
}
return pInstance;
}
}

>

Can I change the pInstance from outside?

>

No, unless you somehow pass a pointer or reference to the variable to
the outside world from the function. But that's such a mind-bogglingly
silly violation of encapsulation that it's not worth considering.

>

If you want this ability, then don't use function-scoped static
variables. Use a static class member instead. If you can't modify the
original class, then consider the possibility that there's a very good
reason why you're not supposed to change this value from the outside.

>
Ed:
>
I don't know why you want to do this, but one way would be
>
static Apple* Instance(bool bReset = false)
{
static Apple* pInstance(NULL) ;
if( bReset )
{
delete pInstance;
pInstance = NULL;
}
else if( ! pInstance )
{
pInstance = new Apple();
}
return pInstance;
>
}
>
BTW, since this is a question on standard C++ (not C++/CLI), you would be better
to ask it in microsoft.publi c.vc.language.
>
--
David Wilkinson
Visual C++ MVP

Thank you!
I just don't want the static variable to be declared as a class
member.

Ed.

[Pavel Minaev]

Re: How to change instance variable value in instance function

On Aug 4, 6:02 am, Ed <seah...@gmail. comwrote:

>If I have a class:
>class Apple
>{
>public:
>    static Apple* Instance()
>    {
>        static Apple* pInstance(NULL) ;
>        if( ! pInstance)
>        {
>             pInstance = new Apple();
>        }
>        return pInstance;
>    }
>}

>

>Can I change the pInstance from outside?

>
I just don't want the static variable to be declared as a class
member.

Why, really?

But anyway, here's another trick:

class Apple
{
static Apple*& pInstance()
{
static Apple* value(NULL);
return value;
}

static Apple* Instance()
{
if( ! pInstance())
{
pInstance() = new Apple();
}
return pInstance();
}
};