Two questions involving STD Lists

This is a discussion on Two questions involving STD Lists within the C++ Programming forums, part of the General Programming Boards category; In my game, i use a list for my entities, among other things. my first question is... is it possible ...

  1. #1
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    Two questions involving STL Lists

    In my game, i use a list for my entities, among other things.

    my first question is... is it possible to declare an extern list? i want to be able to access my list of entities from anywhere in my code (including other source files). example...

    in main.cpp....
    Code:
    list <CBaseEntity *> entities;
    blalbbllah
    blahblah
    blah
    in say, weapons.h....
    Code:
    extern ????????? entities;
    and now in weapons.cpp.....
    Code:
    object_thingy *object = new Object();
    entities.push_back(object);

    and finally my second question, how would i go about getting the total number of elements in a list? for example, i want to check if a list contains over 200 elements, and then delete everything past 200 to save on memory.
    Last edited by IonBlade; 12-09-2003 at 04:15 PM.

  2. #2
    C++ Developer XSquared's Avatar
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    You mean STL lists, right?

    1) No clue.
    2) Use the size( ) method to get the size, then you can use an iterator to get to the 200th element, then use the erase( ) method.
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  3. #3
    Registered User jlou's Avatar
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    Originally posted by IonBlade
    and finally my second question, how would i go about getting the total number of elements in a list? for example, i want to check if a list contains over 200 elements, and then delete everything past 200 to save on memory.
    I'm not sure a list is the best option if you need to iterate to find the 200th element. You'll probably want to access that 200th element randomly - meaning you won't need to loop through all 199 elements before it. I don't know of any efficient way to lop off all elements after 200 in a list container without navigating your way to the 200th element first.

    A better container might be a deque or a vector. You can get an iterator to the 200th element simply by calling at(200) (technically that would be the 201st element). You can then use that iterator with erase/remove. You can push_back on to a deque or vector just like a list, but you cannot insert an element into the middle as efficiently as you can with a list. However, if you don't do any inserts, only push_back, then deque or vector is for you. Or if you use push_front, then deque (not vector) might be better.

    As far as your first question, I don't know any reason why it wouldn't be possible. Are you having problems with it?

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    yeah lol, STL. STD? god now i feel like an idiot damn abbreviations, SDL, STL, STD, same thing..

    As far as your first question, I don't know any reason why it wouldn't be possible. Are you having problems with it?
    i'm just not sure of the syntax. and yes i have tried and i got errors.

    Code:
    extern list entities;
    extern list *entities;
    extern list<CBaseEntity *> entities;
    i have tried all those, and get "syntax error before ';' ".

    sorry if i sound noobish, but i'm just starting with the STL. and that list of 200 elements is what i'm using to keep a console/message log, it's just a list of char strings, and i want to delete past messages when about 200 exist.
    Last edited by IonBlade; 12-09-2003 at 04:19 PM.

  5. #5
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    extern works fine with stl, the only problems you might find is forgetting to actually declare it somewhere, or more than one place. Both don't show up utill linkage though. I would suggest
    Code:
    #ifndef CONLIST_H
    #define CONLINST_H
    #include<deque>
    #include<string>
    typedef std::deque<std::string> conlist_t;
    extern conlist_t entities;
    #endif
    
    //in one, and only one cpp file
    #include"conlist.h"
    conlist_t entities;
    and just let entities expand to it's little hearts desire. If 200 lines is too much to deal with from a user standpoint then wrap deque around your own class that has a limit that when you add a line and size() > limit it pop_front()s when it push_back()s the new line.

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