Is this safe - linked list with multiple types

This is a discussion on Is this safe - linked list with multiple types within the C Programming forums, part of the General Programming Boards category; Originally Posted by pronecracker Because they are related, they are different kinds of shapes in some sort of drawing program. ...

  1. #16
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by pronecracker View Post
    Because they are related, they are different kinds of shapes in some sort of drawing program. And they are possibly stored in order from back to front (Z).
    I'd suggest storing the objects themselves elsewhere, and keeping a void * pointer to the object inside the node. Functions which operate on nodes would cast the void * to the appropriate type based on the value of iType.

    This is basic polymorphism, which is much easier to handle in C++ than in C.

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    Registered User pronecracker's Avatar
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    Quote Originally Posted by brewbuck View Post
    This is basic polymorphism, which is much easier to handle in C++ than in C.
    I know, please don't take my signature too literally I sometimes have thoughts like that, because C seems so simple and I sometimes get confused because of all the functionality of C++.

    But, why is using a void* safer then what I'm doing now? And what about
    Code:
    union {
    Type1 *pType1;
    Type2 *pType2;
    };
    Don't call me stupid.

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    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by pronecracker View Post
    But, why is using a void* safer then what I'm doing now? And what about
    Code:
    union {
    Type1 *pType1;
    Type2 *pType2;
    };
    I'd say it's safer because it's more likely to work on some screwy compiler. The standard indicates that what you are doing SHOULD work, but I wouldn't count on it. The union thing is fine, too. I do something very similar when writing parsers that build parse trees -- each node of the tree is of type Node, with some code which identifies what kind of node it is, and the data specific to that node type is kept in a field of a union.

    If you're going to use a union, no need to use pointers. Just embed the type-specific data directly in there.

  4. #19
    Just Lurking Dave_Sinkula's Avatar
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    Quote Originally Posted by pronecracker View Post
    But, why is using a void* safer then what I'm doing now? And what about
    Code:
    union {
    Type1 *pType1;
    Type2 *pType2;
    };
    Fine details for varying systems.
    http://www.open-std.org/jtc1/sc22/wg...24.pdf#page=59

    Implementations have existed in which pointers to different types have different representations and/or sizes.
    http://c-faq.com/null/machexamp.html
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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