Guaranteeing only one instantiation of a class

This is a discussion on Guaranteeing only one instantiation of a class within the C++ Programming forums, part of the General Programming Boards category; The question I'm working on is: Create a class that represents a printer connection, and that only allows you to ...

  1. #1
    Registered User Sharke's Avatar
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    Guaranteeing only one instantiation of a class

    The question I'm working on is: Create a class that represents a printer connection, and that only allows you to have one printer.

    Can someone give me a clue as to how to implement a class that limits the number of instantiations? I thought of having a class member which I can set to "false" if the static counter is already at 1, but this means the failed object nonetheless exists and seems messy since the client has to test the state of the object on their end. Better to have the object abort its own construction if there is already one in existence. But how do I go about doing this? Clue needed!

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    I don't know is it the correct method to use.
    But you can always use a special constructor function which live outside the scope of the class.

    for example
    Code:
    TheObject & createTheObject()
    {
         static TheObject theobj = new TheObject();
    
         return theobj;
    }
    it will only create the object once no matter how many times you call the function

  3. #3
    and the hat of sweating
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    Do a search for the Singleton Design Pattern.
    "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008

    "the internet is a scary place to be thats why i dont use it much." - billet, 03/17/2010

  4. #4
    Registered User Sharke's Avatar
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    Both good ideas, thanks! I'm reading about the Singleton pattern now. Although I'm not sure it's the solution the author of the book I'm using had in mind at this stage.

  5. #5
    The larch
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    I thought of having a class member which I can set to "false" if the static counter is already at 1, but this means the failed object nonetheless exists and seems messy since the client has to test the state of the object on their end.
    This might also work, but you could throw an exception: the failed object would not exist and the user wouldn't need to test object (just handle the errors).
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

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