Forward declaration of structs?

This is a discussion on Forward declaration of structs? within the C++ Programming forums, part of the General Programming Boards category; Hello, My problem is that I have two structs that need to know about each other. Obviously just having them ...

  1. #1
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    Forward declaration of structs?

    Hello,

    My problem is that I have two structs that need to know about each other. Obviously just having them like this won't work:

    Code:
    struct Requester
    {
    	bool beingServiced;
    	list<Request> requests;
    };
    
    struct Request
    {
    	Requester r;
    	int track;
    };
    This doesn't compile. I think what I need is a "forward declaration" but I'm not sure what that is or how to do one, I just know that it will let me do this I think.

  2. #2
    CSharpener vart's Avatar
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    Code:
    struct Request;
    
    struct Requester
    {
    	bool beingServiced;
    	list<Request> requests;
    };
    
    struct Request
    {
    	Requester r;
    	int track;
    };
    you mean like that?
    I'm not sure that will work for whole objects and not pointers or referencies...
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  3. #3
    The larch
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    It seems that incomplete types are not allowed to be used as list template argument.

    You may need to put Request before Requester, forward declare Requester and replace the r member with Requester* (why does request hold a requester by value?)
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  4. #4
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    Figured it out thanks. First suggestion did the trick.

  5. #5
    C++まいる!Cをこわせ! Elysia's Avatar
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    Is the first even legal? Unless, of course, list only references the struct via pointers or references, of course, and seeing as it is a std::list, that makes sense...
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    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

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  6. #6
    The larch
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    List holds copies of the instances if I'm not mistaken

    In any case, Comeau online says this:

    Code:
    "concept_checks.h", line 555: error: incomplete type is not allowed
        static void _Assignable_requirement_violation(_Type __a) {
                                                      ^
              detected during instantiation of "void
                        _Assignable_concept_specification<_Type>::_Assignable_requi
                        rement_violation(_Type) [with _Type=Request]" at line 8 of
                        "ComeauTest.c"
    Code:
    #include <list>
    struct Request;
    
    struct Requester
    {
    	bool beingServiced;
    	std::list<Request> requests;
    };
    
    struct Request
    {
    	Requester r;
    	int track;
    };
    MinGW and VC++ appear to compile it without errors, though.
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  7. #7
    Master Apprentice phantomotap's Avatar
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    Comeau is just being aggressive.

    The standard says that a template type argument may be an incomplete type. (14.3.1.2)

    Soma

  8. #8
    Cat without Hat CornedBee's Avatar
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    The standard also says that using an incomplete type as the type of a container is undefined behavior. (17.4.3.6p2)
    All the buzzt!
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  9. #9
    Master Apprentice phantomotap's Avatar
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    It does. The rationale is ridiculous. Off hand, I don't know of any implementation where it doesn't work. (This goes for the version of Comeau I have installed. The definition of the containers under certain assumptions are setup to instantiate firewalls which naturally fail even before any functions are invoked.)

    By the by, I do not intend to imply that I think the design here is good.

    Soma

  10. #10
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    It probably isn't. I'm new to C++ and for a project I'm doing I have a list of Requestors each having their own Requests. The Requests have to know where they came from. This seemed the easiest way to do it.

  11. #11
    Kernel hacker
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    Quote Originally Posted by jcafaro10 View Post
    It probably isn't. I'm new to C++ and for a project I'm doing I have a list of Requestors each having their own Requests. The Requests have to know where they came from. This seemed the easiest way to do it.
    Then clearly, you need a pointer to the requestor that it came from, not a copy of the requestor.

    --
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  12. #12
    Master Apprentice phantomotap's Avatar
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    O_o

    With that in mind, the 'Request' object doesn't own its "requesting" object--the 'Requester' class. The 'Request' object only needs a link to the "requesting" object. You want a pointer, or for a preference, given the original post, a weak pointer.

    Soma

  13. #13
    The larch
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    But the requests don't know where they are coming from, do they? Instead they create a totally new and unrelated Requester instance...

    If the contained object need to know about its owner, you do need a pointer (or reference) to it. And then you should be able to forward declare it the other way round...
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

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