Can someone give me some guidance as to how to create a Singleton object in
managed C++ for use in Visual Basic.NET? I can't seem to create one without
explicitly calling some constructor or another, plus I'm getting a
NullReferenceException when I try to call the Instance() routine. How do you
do it?

Thanks

#include "stdafx.h"

#using <mscorlib.dll>

__gc public class Singleton

{

protected:

static Singleton* instance;

Singleton() {}

public:

static Singleton* getInstance();

Singleton* Singleton::getInstance()

{

if (instance == 0)

instance = new Singleton();

return instance;

make sure that Instance() method of your's is delcared as static.
-dec

P.S. helping you solve that problem helped me solve some stupid ass problem
I have been stuck on for like 2 hours...so thanx hehe

-dec

This is perfect code as long as this method is only called on a single
thread.  If the program is at all multithreaded, you definitely want to make
sure a lock is taken before executing the if statement.

You can easily do this by putting the following attribute on the method from
the System::Runtime::CompilerServices namespace.
   [MethodImpl(MethodImplOptions::Synchronized)]

There are probably more efficient design patterns out there, but be
cautious -- some design suggested design patterns don't always work (such as
double checked locking).

Cheerio!

--
Brandon Bray                                          Visual C++ Compiler
This posting is provided AS IS with no warranties, and confers no rights.

Hi Brandon,

You are absolutely right on this. However, wouldn't the easier way be to
make use of type initializers (aka static constructors) instead of locking?

--
Tomas Restrepo

Hi Tomas,
  You're right.  To my chagrin, I completely forgot that static class
constructors were even supported in the language right now.  Doh!

--
Brandon Bray                                          Visual C++ Compiler
This posting is provided AS IS with no warranties, and confers no rights.

How is the static class implemented for this scenario?

can you throw down a code sample? Having a lil trouble conceptualizing it..

cheers
-dec

Sure.  The singleton pattern is enforced by the fact that the CLR guarantees
a static class constructor will be called exactly once.  The code you
originally posted would be written as follows to use this design:

#using <mscorlib.dll>

__gc class Singleton {
protected:
  static Singleton* instance;

  static Singleton() {
    instance = new Singleton();
  }

  Singleton() {}

public:
  static Singleton* getInstance();

Singleton* Singleton::getInstance() {
  return instance;

--
Brandon Bray                                          Visual C++ Compiler
This posting is provided AS IS with no warranties, and confers no rights.

Oh I thought you ment like ...
__gc static class Xxx { };

In any case, I fail to see how making the constructor static changes
anything, multithreaed or not...

If you have a private constructor and a static method which calls this
constructor if and only if it has never been called before, what does it
matter how many threads try to capture the object? Multiple threads won't
create multiple objects =P

To take it furthure...
(if instance == 0) instance = new ClassName();

Is it even possible for a thread to get throgh the if and stop right before
class instantiation only to let another thread start up and complete the if
check and actualy instatiate the object then stopping and the first thread
instatiates again??? I always thought threaed or not, an executable line is
locked. Rather, if a thread starts to execute a line of code it does not
stop until it hits a semi-collon....am I wrong on this?

Thanks for you feedback and helping me to furthure my knowledge
-dec

Decrypted,

Actually, it does.

If the object is a singleton, it sure matters as hell. You might very well
end up creating more than one object, which defeats the whole purpose of
making it a singleton.

You're confusing what a static constructor does, though. The runtime
guarantees that the static constructor will be called exactly once, even in
the presence of multiple threads, the first time a static member of the type
is accessed.

Yep. Very possible.

Not at all. A line of code could generate dozens of individual instructions.
And why lock them, anyway? most apps aren't multithreaded, and so, locking
on them would only cause a performance hit.

Completely :)

--
Tomas Restrepo