Silly Q about circular references

This is a discussion on Silly Q about circular references within the C Programming forums, part of the General Programming Boards category; Hello, I have a silly question about circular references in C (note: not C++): typedef void ( *FSM ) ( ...

  1. #1
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    Silly Q about circular references

    Hello,

    I have a silly question about circular references in C (note: not C++):

    typedef void ( *FSM ) ( CALL_CB* );

    typedef struct _fsm_cb
    {
    FSM fsm_entry;
    } FSM_CB;

    typedef struct _call_cb
    {
    FSM_CB *fsm;
    } CALL_CB;

    int main()
    {
    CALL_CB* call_cb;

    return 1;
    }
    This can't compile because FSM_CB is referencing CALL_CB and vice versa:
    test.c:4: syntax error before '*' token

    I know that C++ has a solution to this, by simply declaring the second class before the first:
    class Second;
    class First
    {
    Second* m_second;
    };
    class Second
    {
    First* m_first;
    };

    main ()
    {
    First* aFirst;
    Second* aSecond;
    }

    Can anyone help?

    Thanks,
    Paul

  2. #2
    Code Goddess Prelude's Avatar
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    >I know that C++ has a solution to this, by simply declaring the second class before the first
    C has the same solution.
    My best code is written with the delete key.

  3. #3
    Registered User SKeane's Avatar
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    Code:
    struct _call_cb;
    
    typedef void ( *FSM ) ( struct _call_cb* );
    
    typedef struct _fsm_cb
    {
    
        FSM fsm_entry;
    
    } FSM_CB;
    
    typedef struct _call_cb
    {
    
        FSM_CB *fsm;
    
    } CALL_CB;
    
    int main()
    {
    
        CALL_CB* call_cb;
    
        return 1;
    
    }
    Possibly
    Last edited by SKeane; 10-04-2006 at 07:41 AM.

  4. #4
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    I had tried that, still doesn't work:

    struct _call_cb;

    typedef void ( *FSM ) ( CALL_CB* );

    ...

    test.c:3: syntax error before '*' token


    Any other ideas?
    Paul

  5. #5
    Registered User SKeane's Avatar
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    Try reading what I actually posted?

    I've highlighted the bit you missed.

  6. #6
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    Oops, thanks SKeane.

    That worked! Great!
    Paul

  7. #7
    Registered User SKeane's Avatar
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    Not saying it'll work mind!

  8. #8
    Frequently Quite Prolix dwks's Avatar
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    You could also put the typedef beforehand (at least I think so, and Dinkumware does too):
    Code:
    typedef struct _call_cb CALL_CB;
    
    typedef void ( *FSM ) ( CALL_CB* );
    
    typedef struct _fsm_cb
    {
    
        FSM fsm_entry;
    
    } FSM_CB;
    
    struct _call_cb
    {
    
        FSM_CB *fsm;
    
    };
    
    int main()
    {
    
        CALL_CB* call_cb;
    
        return 1;
    
    }
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
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  9. #9
    Cat without Hat CornedBee's Avatar
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    Yes, that's valid. I would have posted it, but I was too lazy.
    All the buzzt!
    CornedBee

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