ptr_container and pointers

This is a discussion on ptr_container and pointers within the C++ Programming forums, part of the General Programming Boards category; Hello I have a ptr_container: Code: typedef boost::ptr_vector<some_object> my_type; my_type m_vector; now I make pointer to it: Code: my_type *p ...

  1. #1
    l2u
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    ptr_container and pointers

    Hello

    I have a ptr_container:

    Code:
    typedef boost::ptr_vector<some_object> my_type;
    my_type m_vector;
    now I make pointer to it:

    Code:
    my_type *p = &m_vector;
    Now I want to have random access to m_vector by using pointer p:

    Code:
    my_type::size_type st;
    for( st = 0u; st != p->size(); ++st ) {
    //now how to call a function inside some_object ?
           p[st]->some_function();
    }
    How should I do this?
    Thanks for help

  2. #2
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    I believe operator[] returns a reference for ptr_vector. Since p is a pointer, dereference it first before using the [] operator, then use the . to access the function of the object it returns.
    Code:
    (*p)[st].some_function();
    
    // or
    
    p->operator[](st).some_function();

  3. #3
    Cat without Hat CornedBee's Avatar
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    That's the gist of it. Has nothing to do with ptr_vector specifically, just generally with the fact that overloaded operators apply to the direct type of their arguments, and in the case of p, the direct type is my_type*, which is not what you want.

    The fact that overloaded operators do not apply to pointers (and that it would be unintuitive and error-prone if they did) was one of the main reasons references were introduced to the language.
    All the buzzt!
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  4. #4
    l2u
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    Hello

    I have one more question:

    In case I have:

    Code:
    class base {
    public:
       virtual void function() {
       //do something
       }
    };
    
    class object : public base {
       virtual void function() {
       //do something
       }
    };
    
    typedef boost::ptr_vector<base> my_type;
    my_type m_vector;
    m_vector.push_back(new object());
    my_type *p = &m_vector;
    
    my_type::size_type st;
    for( st = 0u; st != p->size(); ++st ) {
           object *ptr = static_cast<object *>((*p)[st]);
    }
    How should I cast base pointer to object pointer?
    Is there any website where I can read more about that kind of dereferencing and casting? Im really not good with it.

    Thanks for help again

  5. #5
    Cat without Hat CornedBee's Avatar
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    Why do you insist in having a pointer to the ptr_vector, anyway? It's just inconvenient and distracts from the important part of the problem.

    OK, the rules are:
    1) If you're absolutely, absolutely sure that the object in question is of the derived class, use a static_cast. Actually, Boost has a BOOST_STATIC_DOWNCAST or something like that that does a static_cast in release mode and a checked dynamic_cast in debug mode, which is a better idea.
    2) If you're not sure, do a dynamic_cast and check the result. (A dynamic_cast on references throws bad_cast if the cast is disallowed; on pointers it gives a NULL pointer.)
    3) In general, try to avoid downcasting and prefer virtual functions.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  6. #6
    l2u
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    The library I have to work with returns a pointer to ptr_vector instead of a reference. No idea why though.

  7. #7
    Cat without Hat CornedBee's Avatar
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    So dereference it and assign it to a reference. After you've made sure you're not responsible for deleting the thing, that is.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  8. #8
    l2u
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    Doesnt that mean copying it?

  9. #9
    C++ Witch laserlight's Avatar
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    There is no copying involved if you assign it to a reference.
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  10. #10
    l2u
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    What is the right way to assign it to a reference?
    my_type ref = &object; ?

  11. #11
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by l2u View Post
    Code:
    my_type::size_type st;
    for( st = 0u; st != p->size(); ++st ) {
    //now how to call a function inside some_object ?
           p[st]->some_function();
    }
    Use p[st].some_function() instead.

  12. #12
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by l2u View Post
    What is the right way to assign it to a reference?
    my_type ref = &object; ?
    No.

    Code:
    my_type &ref = object;

  13. #13
    l2u
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    Is this case, object is a pointer, right?

  14. #14
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    no.
    Code:
    my_type object;
    my_type &ref = object;
    http://www.cprogramming.com/tutorial/references.html

  15. #15
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    If you have a pointer, dereference the pointer. You dereference with * not &.

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