Accessing main form from functions in other classes

This is a discussion on Accessing main form from functions in other classes within the C++ Programming forums, part of the General Programming Boards category; I'm trying to draw a line on a form through a function which is in a class that I created ...

  1. #1
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    Nov 2004
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    1

    Accessing main form from functions in other classes

    I'm trying to draw a line on a form through a function which is in a class that I created inside another .h and .cpp file. But I cannot give it access to the main form or it's controls. Is there any way that I can pass the form into the functions of the class or a way that I can access the main form or maybe just a picturebox on the form in which I can draw too. And if so, how can I declare a temporary form / picturebox inside the class which I can then assign to the form / picturebox of the main form to?

    PJ

  2. #2
    Super Moderator VirtualAce's Avatar
    Join Date
    Aug 2001
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    9,596
    This sounds like VB except for the references to .h and .cpp. In this instance you may want to make class A a friend of class B. I don't normally recommend this and normally using friend indicates to you that your class structure is seriously flawed, but there are certain times when a friend is the only way to get the job done. Usually those instances are when you want class A to manage all instances of class B, but you don't want the user to be able to explicitly instantiate class B. They must go through class A to instantiate class B. Of course then class A would need to be a singleton class to avoid more issues.

    But since A and B do not have any prior knowledge of each other and are not derived from each other this is sometimes the only way.

    Code:
    class B;
     
    class A
    {
    static friend class B;
    static vector<B> BList;
    public: 
    	A () {};
    	static unsigned int CreateB(void)
    	{
    	 B *temp=new B;
    	 BList.push_back(temp);
    	 ..set unique ID for new B
    	 return ID;
    	}
    	static B GetB(unsigned int _ID) {return BList[_ID];};
    private:
       //Hide copy constructor to prevent misuse and/or improper implicit instantiation 
       //Just in case somehow someone does get access - always assert as false
       //Fail in all cases
       A(A& obj) {ASSERT(false);};
    };
     
    class B
    {
      private:
    	 B() {}
    };
    I coded this while sitting here so I'm fairly sure it doesn't work as shown. But it shows the general principle where a friend is pretty much the only way to get the job done.
    Last edited by VirtualAce; 11-05-2004 at 08:41 AM.

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