Vectors + Classes = Confusing

This is a discussion on Vectors + Classes = Confusing within the C++ Programming forums, part of the General Programming Boards category; Not quite... Rather, it is another template argument when you create the vector. The beauty of templates (ignoring really cool ...

  1. #46
    Toaster Zach L.'s Avatar
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    Not quite... Rather, it is another template argument when you create the vector. The beauty of templates (ignoring really cool template metaprogramming) is that you can create user customizable code that is standard (term used loosely, but it still holds in the strict sense as well).

    *edit *
    Look at SGI's documentation (my favorite source for STL docs) here: http://www.sgi.com/tech/stl/stl_index_cat.html

    The section at the bottom is... Drum roll, please.... Allocators.
    Last edited by Zach L.; 12-15-2004 at 07:53 PM.
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  2. #47
    Guest Sebastiani's Avatar
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    >> is STL still STL if it has been 'enhanced' by the programmer

    >> STL was designed to be enhancable by the programmer

    that's not true at all! the STL *is* designed to be EXTENSIBLE, meaning you can and should derive classes from it whenever you need to. but that's not the same as tinkering with internal code (a very bad idea (tm)).
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  3. #48
    Toaster Zach L.'s Avatar
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    Changing allocators, however, isn't tinkering with the internal code of the STL implementation.

    (I realize you weren't implying that, Sebastiani, I just thought I'd clarify that point.)
    The word rap as it applies to music is the result of a peculiar phonological rule which has stripped the word of its initial voiceless velar stop.

  4. #49
    Epo
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    Holy pandemonium batman! I leave for one day of exams and things just go nuts.

    Code:
    >> I'm not sure that clear has to be called.
    It doesn't. If you have objectinstances in a container, the destructors for those objects
    will be called when clear() is called or when the container's destructor is called.
    Good to know, thanks for clearing that up. (Way back when)

    Code:
    Illustration of serious memory/resource leak with vectors:
    From back on the first page. I think that's what my initial problem was along the lines of, but I've removed the vector that was causing the problem and reworked it now.

    Code:
    Personal Opinion Follows:
    If you are going to use methods with structs then you should rename it to class and just declare the public part.
    I like that, and it makes it a bit more clear as to what's being used as what. I think I'll take the time to re-structure a touch.

    As for everything else...whew, right over my head, but sounds like there're some serious issues you guys are in need of solving, so I'll leave you to it I've quickly scanned most of the replies, and I'll do a more thorough read, but from the looks of it, there's quite a bit beyond me and my tiny problems.

    Thanks for all your suggestions and help. Now, carry on.

  5. #50
    Banned master5001's Avatar
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    Advantages to using STL: rapid development, extensive features, thoroughly tested
    Advantages to using your own (if done correctly): performance gain, a class completely customized for what you need

    Truthfully, unless you are doing something that deals with raw RAM or virtual memory then I see no reason to be toying with making your own.

  6. #51
    Registered User jlou's Avatar
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    Quote Originally Posted by Sebastiani
    meaning you can and should derive classes from it whenever you need to
    You've got to be careful with this, too. You should not derive from most STL classes, they were not meant to be base classes (like string, vector, etc.). You can and should extend their functionalities by using non-default template parameters like allocators, comparison functions, etc.

  7. #52
    Epo
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    This is still to do with vectors, so I'll just post here to avoid starting up a new thread.

    if I declare a vector of a class, I.e.:
    Code:
    class MyClass
    {
    public:
    	void GoDoIt(void)
    	{
    		//In here is where my
    		//question refers to.
    	}
    };
    
    std::vector<MyClass> Things;
    When MyClass::GoDoIt() has been called, and I'm in the body of that function, is it possible to be able to tell which element of the vector actually called that function (was it Things[1], Things[4]...)? Also, is there a way to tell how many other elements exist?...

    Without the need to pass those values in as parameters into the function.

    So, pretty much, does an element "know" which number, of how many, it is?

  8. #53
    VA National Guard The Brain's Avatar
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    is it possible to be able to tell which element of the vector actually called that function
    If you make your function call like this:
    Code:
    Things[4].GoDoIt( );
    Then you know which element of the MyClass vector of classes called the function.

    While operating inside the body of your function definition.. any changes you make will have a global effect over the entire vector of classes.





    Also, is there a way to tell how many other elements exist?
    Code:
    Things.size( );
    Last edited by The Brain; 12-17-2004 at 04:51 PM.
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  9. #54
    Carnivore ('-'v) Hunter2's Avatar
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    >>So, pretty much, does an element "know" which number, of how many, it is?
    From within the body of the function being called, there are 2 ways I can think of, and neither one is pretty: One is, tell it which element it is, by passing an extra parameter. The other is, if it has access to the vector that it's contained in, you can:
    Code:
    for(int index = 0; index < Things.size(); ++index)
    {
       if(&(Things[index]) == this)
    	  break;
    }
    //Now index is the index in the vector that the object is at.
    As you can see in my code, I used Things.size() to find out how many elements there are in Things.
    Just Google It. √

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  10. #55
    Banned master5001's Avatar
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    Ok there is nothing magical about a vector. It works in a similar manner to a regular array. Does an int in an integer array know which index it is? Of course not. Do you really need for the object to know which index it is?

    Side note: iterators are more suitable in the above code.

  11. #56
    Carnivore ('-'v) Hunter2's Avatar
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    I know, but it would have been harder to understand and would have ran contrary to:
    Ok there is nothing magical about a vector. It works in a similar manner to a regular array.
    Although you and I know better, it would seem to the average newbie that iterators are VERY different from anything involving arrays.
    Just Google It. √

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  12. #57
    Epo
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    I was hoping that they would be some sort of "super-array" that writes my program for me...guess not.

    Yeah, and it makes sense now, I'll just stick to passing the variables as arguements. It's not really that much extra work anyways.

    Thanks guys

  13. #58
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    >> You should not derive from most STL classes, they were not meant to be base classes (like string, vector, etc.).

    and why not?
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  14. #59
    Cat without Hat CornedBee's Avatar
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    They don't have virtual destructors, for one. This means that any such object deleted through a base pointer will invoke undefined behaviour. (So feel free to derive privately from them.)
    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

  15. #60
    Registered User jlou's Avatar
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    Yes, I should definitely have said "publicly derive from". It is really a general rule, not an absolute one (in fact Codeplug debated against such a "rule" with me a long time ago). If you know that you won't be using the class polymorphicly, and you have a good reason to do so, then it probably isn't so bad. However, usually the reason for doing so is to extend the functionality of the class, which can be done in other, better, ways.

    [EDIT] - Ha! Codeplug remembers that conversation.
    Last edited by jlou; 12-18-2004 at 04:46 PM.

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