class prototypes for cross-referencing

This is a discussion on class prototypes for cross-referencing within the C++ Programming forums, part of the General Programming Boards category; The compiler yells at me when I try to make use of a definition in the code above where it ...

  1. #1
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    class prototypes for cross-referencing

    The compiler yells at me when I try to make use of a definition in the code above where it is defined.
    Code:
    class type1
    {
      type2 next;  //It hasn't seen type2 yet, so it shouts at me here
    };
    
    class type2
    {
      type1 previous;
    };
    Is there a way to prototype a class before you define it, like there is with functions, so that I can let the compiler know that type2 is there for type1?

  2. #2
    and the hat of int overfl Salem's Avatar
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    You can't have classes which have mutually recursive members.

    But you can have pointers to mutually recursive members.
    Code:
    class type2;  // a forward declaration for a class
    class type1
    {
      type2 *next;  //It hasn't seen type2 yet, so it shouts at me here
    };
    
    class type2
    {
      type1 *previous;
    };
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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    thanks

  4. #4
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    It got rid of the compiler error at first, but now that I try to actually use it, it still yells at me. It tells me that my use of the undefined class is invalid. Here's what I'm trying to do more specifically.

    I have the classes person and room, and I want the person to have a variable referencing the room it is.
    Code:
    class person
    {
      public:
      room *loc;
      unsigned short int x, y, z;
      person(void)
      {
        x = 0; y = 0; z = 0;
        loc = &rooms[x][y][z];
        loc->enter();
      }
    }
    And for room.enter() I have
    Code:
    class room
    {
      public:
      void enter(PC *PC = NULL)
      {
        if(PC == NULL) PC = &you //you being a variable that keeps track of the main person
        PC->loc = this object
        //I don't know how to reference the source, but that's something I already had a different post asking about
      }
    }

  5. #5
    ZuK
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    You cannot define the functions that reference the pointers to the forward declared class inside the class. One way would be to define them in separate implementation files.
    If you still want to have them inline you could implement them after declaration of the classes using the inline keyword.
    e.g.
    Code:
    class type2;  // a forward declaration for a class
    class type1
    {
      type2 *next;  //It hasn't seen type2 yet, so it shouts at me here
      void do_sth_widh_next();
    };
    
    class type2
    {
      type1 *previous;
      void set_prev( type1 * a ){
         previous= a;
      }
    };
    
    inline void type1::do_sth_widh_next() {
      next->set_prev( this );
    }
    Kurt
    Last edited by ZuK; 03-19-2006 at 03:21 PM.

  6. #6
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    It all seems to be working now. Thanks much.

    Is there any way to tag a post as solved on this board? I know there are on some, and I'm just wondering so I have a way to let people know it's solved without needing to bump the post up.

  7. #7
    and the hat of int overfl Salem's Avatar
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    > Is there any way to tag a post as solved on this board?
    Nope - people just say thanks and the post gets abandoned - if it's lucky.

    People refer to it from time to time (if they can be bothered to search), and just occasionally, some moron will come along and bump threads some years later with a vacuous "me too" comment.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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