Struct inside a Struct inside a Struct?!?

This is a discussion on Struct inside a Struct inside a Struct?!? within the C++ Programming forums, part of the General Programming Boards category; i seem to be having problems with some code.. and to be honest i can't figure out why... i won't ...

  1. #1
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    Question Struct inside a Struct inside a Struct?!?

    i seem to be having problems with some code.. and to be honest i can't figure out why... i won't post my code but i'll post something similar...

    Code:
    // One.h
    
    #ifndef ONE_H
    #define ONE_H
    
    #include "Two.h"
    
    struct One
    {
      int stuff;
      One *next;
      Two *list;
    }
    
    #endif
    Code:
    // Two.h
    
    #ifndef TWO_H
    #define TWO_H
    
    #include "One.h"
    
    struct Two
    {
      unsigned int otherStuff;
      Two *next;
      One *back; // THis is where the error occurs.
    }
    
    #endif
    What i do is make a pointer to a One struct, however the error pops up at where i designated.. it basically won't let me define a [b]One[b] inside a Two struct.. why can't i make a struct point to another struct and that struct point back to the first type of struct... (making a linked list of linked lists)

  2. #2
    Open to suggestions Brighteyes's Avatar
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    You may have to use a forward declaration:
    Code:
    struct B; // Make the name B known
    
    struct A {
        A* a;
        B* b;
    };
    
    struct B {
        A* a;
        B* b;
    };
    p.s. What the alphabet would look like without q and r.

  3. #3
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    You might need to typedef the structs, or at least refer to them as structs.
    Code:
    struct One
    {
      int stuff;
      struct One *next;
      struct Two *list;
    }
    Code:
    struct Two
    {
      unsigned int otherStuff;
      struct Two *next;
      struct One *back; 
    }
    Other suggestions are to not put them in different header files, since they are dependent on each other. I don't know if you can declare variables as externs in C++ or not, but that could also help.

  4. #4
    pronounced 'fib' FillYourBrain's Avatar
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    This is C++ so no, the struct keyword isn't necessary even when you didn't do the typedef trick (someone pointed that out to me recently).

    also, since it's C++ we should be using classes anyway.
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  5. #5
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    Greetings,

    C++ is all about classes! Besides, structs are classes in C++, the only difference (apperently) is that in struct's the members default to public where in classes it defaults to private.

  6. #6
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    Ah, yes. It's been a while since I've programmed anything with classes, so I kinda forgot what I was doing on for a second. Anyway, I don't know exactly what the error message says, but I think Brighteyes is on the right track with having to somehow delcare the struct as a known variable beforehand.

    As an aside: Even though something may not be necessary, my experience tells me that programs sometimes work better if it's done anyway.

  7. #7
    pronounced 'fib' FillYourBrain's Avatar
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    syntax changes don't make programs run different. logic changes do.
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  8. #8
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    ah

    ah quick replies.. nice.. i'll try some of ur guys suggestions and see if i can get it to work...

    yeah i tried to put both structs in same file but since i never used forward declarations before it makes it kinda hard :P .. i gotta remember forward decs'... could be useful... yeah there is externs in c++ too.. might give that a whirl as well..

    i've never seen any code written where u refer to the datatypes by structs struct One *next; but if that works that could be pretty cool too..



    i'lll get back to ya'll if i run into more problems... thnx for the help

  9. #9
    pronounced 'fib' FillYourBrain's Avatar
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    Re: ah

    Originally posted by tegwin
    i've never seen any code written where u refer to the datatypes by structs struct One *next; but if that works that could be pretty cool too..
    that's the way it had to be done in C
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

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