Reference to singleton as member

Shouldn't it be possible to keep a reference to a singleton class as a member? For example:

Code:

---

class Single
{
public:
  static Single* instance(){
                            if(inst==0) inst = new Single;
                            return inst;
                          }
private:
  static Single* inst;
};
Single* Single::inst = 0;


class Foo
{
public:
  Foo():x(Single::instance()){}
private:
  Single& x;
};

---

The errors I get:

Quote:

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main.cpp(20): error C2439: 'Foo::x' : member could not be initialized
main.cpp(22) : see declaration of 'Foo::x'
main.cpp(20): error C2354: 'Foo::x' : initialization of reference member requires a temporary variable

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The compiler is telling me I can't initialize a reference to a temporary variable, but it isn't really temporary is it? Maybe I should just use a pointer for the member variable, or is there a much better way to do this?

You should use a pointer instead of a reference, becaue it is a singleton class, you don't want to have ownership to the singleton instance.

By the way, the sample you've is just a quick write-up, right?
I see there's some syntax that is incorrect.

Also, keep in mind that if you're doing multi-threaded programming, you need to make sure there will be no two threads accessing the getInstance() get the same time.

Most people apply a mutex in the getInstance() function to synchronize the function.

>>You should use a pointer instead of a reference, becaue it is a singleton class, you don't want to have ownership to the singleton instance.

Hmm I don't quite see what difference it makes...In any case, I'll use a pointer--it doesn't really seem to make much of a difference as far as I can see

>>By the way, the sample you've is just a quick write-up, right?
I see there's some syntax that is incorrect.

Yes but I don't see the incorrect syntax :confused:

And I'm not doing anything multithreaded

Try;

Code:

---

  Foo(): x(*Single::instance()){}

---

Initialise a reference with a reference, not with a pointer. Either that, or change Single::instance() so it returns a reference, not a pointer.

Quote:

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Originally Posted by JaWiB

>>You should use a pointer instead of a reference, becaue it is a singleton class, you don't want to have ownership to the singleton instance.

Hmm I don't quite see what difference it makes...In any case, I'll use a pointer--it doesn't really seem to make much of a difference as far as I can see

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If you own a reference to the singleton instance, basically it means you've the ownership of that piece of memory. You can set the reference to point to null. If some other classes come in later and would like to obtain an instance of the singleton, then they'll get a bad reference.

Quote:

---

Originally Posted by JaWiB

>>By the way, the sample you've is just a quick write-up, right?
I see there's some syntax that is incorrect.

Yes but I don't see the incorrect syntax :confused:

And I'm not doing anything multithreaded

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Code:

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if(inst==0) inst = new Single; // your compilation will fail here.

if(inst==0) inst = new Single(); // aahh... much better :)

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Quote:

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Originally Posted by grumpy

Initialise a reference with a reference, not with a pointer. Either that, or change Single::instance() so it returns a reference, not a pointer.

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Ah...yeah that might work better heh

>> If you own a reference to the singleton instance, basically it means you've the ownership of that piece of memory. You can set the reference to point to null. If some other classes come in later and would like to obtain an instance of the singleton, then they'll get a bad reference.

This is not true. You cannot set a reference to null. You cannot set the object referred to by the reference to null, as it will not affect any other references to that object. At worst, you can delete the Single object, but you can do that with a pointer as well, and presumably the destructor is private or protected which prohibits that anyway.

>> if(inst==0) inst = new Single; // your compilation will fail here.

That syntax is valid.