Copy constructors and private constructors

This is a discussion on Copy constructors and private constructors within the C++ Programming forums, part of the General Programming Boards category; I'm creating a few wrapper classes for the SDL library in anticipation of making a game or something, right now ...

  1. #1
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    Copy constructors and private constructors

    I'm creating a few wrapper classes for the SDL library in anticipation of making a game or something, right now i'm wrapping the video init code. class display has private constructors, public named constructors return a pointer to a display class. Only one display class can be initilized at a time.

    Question is this; do I really need a copy constructor/could the copy constructor ever be called, if all the user ever gets is a pointer to a display object?

    If a copy constructor could be called, how can I handle it? Only one object can be initilized at a time... copy the current object and kill it? (delete object;) throw an exception?

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    You say a SDL wrapper...

    My SDL wrapper ZEngine has tons of work into it, email me to talk about working on it or just using it. Depending on how you set it up you may never need a copy constructor.. I'd recommend a singleton approach.

    Singleton is a class that there can only be one instance of, and works very well in the design of the core part of an engine.

    james@conceptofzero.net, we can talk SDL. (ZEngine has tons of stuff, SDL addons, INI reading, loading data from zip files, soon a GUI)

  3. #3
    Guest Sebastiani's Avatar
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    Well, you might define a do-nothing copy constructor to inhibit any default behaviour.

    [quote]
    If a copy constructor could be called, how can I handle it? Only one object can be initilized at a time... copy the current object and kill it? (delete object throw an exception?
    [quote]

    Why would you want to? Anyway, nothing stopping this:

    Display * one = Get(); //..."the pointer"
    char * chunk = new char[sizeof(Display)];
    Diplay * two = (Display*)chunk;
    memcpy(two, one, sizeof(Display));
    two->blit();
    Code:
    bool fun(bool value)
    {
        return std::pow(std::exp(1), std::complex<float>(0, 1) 
        * std::complex<float>(std::atan(1)*(1 << (value + 2))))
        .real() > 0;
    }

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    Yes, a singleton is exactly what I need. Thanks for the terminology, it yeilded many search results. Most examples use either an unnamed class or a static reference count, I was using the reference count prior to all of this.

    I'd like to take a look at your library, I'll probably learn something from it. I'll give you an email later on. Do you have either ICQ or MSN Messenger?

    I searched for SDL wrapper libraries before starting, one interesting one came up; SEL (http://sel.sourceforge.net I think) but it looked as if it was written by beginners... for instance the display class didn't have any saftey mechanisms on it like i'm trying to implement.

  5. #5
    &TH of undefined behavior Fordy's Avatar
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    Singletons are real handy for situations where only 1 object can be in usage.....you can impose a reference counting control, but if the count can only be 1, then here's a simpler method;

    Code:
    #include <iostream>
    
    class foobar{
    	foobar(){}//Private...so only foobar::GetFoobar can use
    	foobar(const foobar&);//as above
    public:
    	void Sing(){std::cout << "lalalalalala" << std::endl;}
    	
    	static foobar& GetFoobar();//static member function to access singleton
    };
    
    foobar& foobar::GetFoobar(){
    	static foobar f;//this is created once on the first call to func
    	return f;//return reference
    }
    
    int main(){
    
    	//foobar a; \\cant create instance
    	
    	foobar& b = foobar::GetFoobar();//can get and use reference	
    	b.Sing();
    	
    	foobar::GetFoobar().Sing();//Can get and use with single call
    	
    }
    As you see, the constructors are all private.....so the only functions with access to create a foobar object are the static member functions of foobar itself.....so if you define a static member func that creates a static object, it will be created only once when the function is first called....you can then use that function and all it will do is return the reference to the single object.......quite nice and clean...

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    Thanks for the help guys, I chose a simple reference count for this. It seems to be working perfectly. I created a class, singleton and subclassed display with it; as I can see needing more singletons in the future.
    Last edited by Eibro; 11-24-2002 at 02:35 PM.

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