Why isn't my class working?

This is a discussion on Why isn't my class working? within the C++ Programming forums, part of the General Programming Boards category; When I try to compile this: Code: class my_class; class my_class { my_class ply; char board[10]; }; Dev-C++ returns the ...

  1. #1
    Registered User Queatrix's Avatar
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    Question Why isn't my class working?

    When I try to compile this:
    Code:
    class my_class;
    class my_class
    {
       my_class ply;
       char board[10];
    };
    Dev-C++ returns the error:
    Quote Originally Posted by Dev-C++
    field `ply' has incomplete type
    Why? And how can I fix it?

  2. #2
    The superhaterodyne twomers's Avatar
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    Code:
    class my_class;
    class my_class
    {
       my_class *ply;
       char board[10];
    };
    ? I think it needs to be of pointer type.

    If you're interested I get this error on visual studio -

    error C2460: 'my_class::ply' : uses 'my_class', which is being defined
    Last edited by twomers; 09-17-2006 at 11:48 AM.

  3. #3
    Registered User Queatrix's Avatar
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    Thanks, that worked.

    But why a pointer to work?

  4. #4
    Cat
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    Quote Originally Posted by Cool-August
    When I try to compile this:
    Code:
    class my_class;
    class my_class
    {
       my_class ply;
       char board[10];
    };
    Dev-C++ returns the error:

    Why? And how can I fix it?
    You can't do something like this.

    Think about it this way:

    sizeof(my_class) = sizeof(ply) + sizeof(board);
    sizeof(my_class) = sizeof(my_class) + sizeof(char[10]);

    You need to make ply a pointer (or a reference but that would be very difficult to use); essentially my_class objects would have infinite size in your original code because they'd be 10 characters larger than themselves.

    Imagine trying to make a semi truck+trailer, whose trailer was large enough to hold an identical semi truck+trailer plus some additional cargo. Impossible, no?

    You would get into even more craziness than that, though -- for example, when you construct a my_class object, it would automatically construct another my_class object for ply, which would construct another my_class object, which would keep going on infinitely.
    Last edited by Cat; 09-17-2006 at 11:59 AM.
    You ever try a pink golf ball, Wally? Why, the wind shear on a pink ball alone can take the head clean off a 90 pound midget at 300 yards.

  5. #5
    The superhaterodyne twomers's Avatar
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    >> But why a pointer to work?

    I won't be able to explain (coherently), nearly as good as someone else, so I'll leave it to them.

  6. #6
    Registered User Queatrix's Avatar
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    I see.

    BTW: twomers, you posted late.

  7. #7
    The superhaterodyne twomers's Avatar
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    Quote Originally Posted by Cool-August
    BTW: twomers, you posted late.
    You're right, but I got distracted by something shiny

  8. #8
    Registered User Queatrix's Avatar
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    Quote Originally Posted by twomers
    You're right, but I got distracted by something shiny
    Your sig?

  9. #9
    The superhaterodyne twomers's Avatar
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    Quote Originally Posted by Cool-August
    Your sig?
    You're just jealous. But yes

  10. #10
    Registered User Queatrix's Avatar
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    >> You're just jealous. But yes

    Why would I be jealous of "Site down for exciting upgrades!"?

  11. #11
    The superhaterodyne twomers's Avatar
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    I meant the colors! Honestly Site's been updated to use MySQL, isn't that exciting?

  12. #12
    Registered User Queatrix's Avatar
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    Quote Originally Posted by twomers
    I meant the colors! Honestly Site's been updated to use MySQL, isn't that exciting?
    Why don't you say your upgrading to MySQL on the website?

    And my sigs got colors ... kind'a ... sort'a ... see?

  13. #13
    vae victus! skorman00's Avatar
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    You can use a pointer because the size of a pointer is always known at compile time. You also have to call new on the pointer to call the constructor. Creating an object within itself would cause the constructor to recurse into itself infinitely ( as Cat pointed out ).

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