Reference to singleton as member

This is a discussion on Reference to singleton as member within the C++ Programming forums, part of the General Programming Boards category; Shouldn't it be possible to keep a reference to a singleton class as a member? For example: Code: class Single ...

  1. #1
    carry on JaWiB's Avatar
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    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:
    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
    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?
    "Think not but that I know these things; or think
    I know them not: not therefore am I short
    Of knowing what I ought."
    -John Milton, Paradise Regained (1671)

    "Work hard and it might happen."
    -XSquared

  2. #2
    Distributed Programming beyonddc's Avatar
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    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.
    Last edited by beyonddc; 01-27-2006 at 10:12 PM.
    -dc

  3. #3
    carry on JaWiB's Avatar
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    >>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

    And I'm not doing anything multithreaded
    "Think not but that I know these things; or think
    I know them not: not therefore am I short
    Of knowing what I ought."
    -John Milton, Paradise Regained (1671)

    "Work hard and it might happen."
    -XSquared

  4. #4
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    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.

  5. #5
    Distributed Programming beyonddc's Avatar
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    Quote 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
    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

    And I'm not doing anything multithreaded
    Code:
    if(inst==0) inst = new Single; // your compilation will fail here.
    
    if(inst==0) inst = new Single(); // aahh... much better :)
    -dc

  6. #6
    carry on JaWiB's Avatar
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    Quote 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.
    Ah...yeah that might work better heh
    "Think not but that I know these things; or think
    I know them not: not therefore am I short
    Of knowing what I ought."
    -John Milton, Paradise Regained (1671)

    "Work hard and it might happen."
    -XSquared

  7. #7
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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.

    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.

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