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POstd::sort going overboard
 

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    <p>I'm trying to sort a vector of objects using a predicate function and I'm getting some segfaults...</p> <p>I have a class <code>Item</code> and a list of Items in a <code>vector&lt; Item &gt; _items</code>. I needed to sort it according to a display order (numerical member of the class) and I just called a simple sort with a predicate function.</p> <pre><code>sort(_items.begin(), _items.end(), sort_item_by_display_order); </code></pre> <p>where the predicate function is </p> <pre><code>bool sort_item_by_display_order (Item i, Item j) { return i.GetDisplayOrder()&gt;j.GetDisplayOrder(); } </code></pre> <p>and GetDisplayOrder is</p> <pre><code>int Item::GetDisplayOrder() { return display_order; } </code></pre> <p>but... I got some segfaults while doing this. I then added a counter to the predicate function to check how many times it was called and I discovered that when this crashed the counter was bigger then the size of the vector.</p> <p>After some reading I changed the code to use iterators instead of using the .begin() and .end() (Shouldn't this be the same?!)</p> <p>So what I have now is</p> <pre><code>vector&lt;Item&gt;::iterator it_start, it_end; it_start = _items.begin(); it_end = _items.end(); sort(it_start, it_end, sort_item_by_display_order); </code></pre> <p>with the same predicate function.</p> <p>And now it doesn't crash, but... for most of the sorting I do I get more iterations then the size of the vector I am sorting (which is probably normal)</p> <p>So... What is the difference between calling sort with <code>_items.begin()</code> or <code>_it_start</code>. From what I can tell they are the same right?!</p> <p>One more note. <code>Item</code> is a simple base class declared as</p> <pre><code>class Item { private: (...) public: (...) } </code></pre> <p>As reference I used <a href="http://www.cplusplus.com/reference/algorithm/sort/" rel="nofollow">http://www.cplusplus.com/reference/algorithm/sort/</a> and <a href="http://www.codeguru.com/forum/showthread.php?t=366064" rel="nofollow">http://www.codeguru.com/forum/showthread.php?t=366064</a>.</p> <p>In the second link they add a const and &amp; to the predicate function arguments which would make my function something like this</p> <pre><code>bool sort_item_by_display_order (const Item&amp; i, const Item&amp; j) { return i.GetDisplayOrder()&gt;j.GetDisplayOrder(); } </code></pre> <p>but I get a compiler error:</p> <pre><code>Item.cpp|1485|error: passing `const Item' as `this' argument of `int Item::GetDisplayOrder()' discards qualifiers| </code></pre> <p>arghhh... The question is... What am I doing wrong?</p>
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    1. USChristian Rau
    1. USAndré Moreira
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    1. COI would be wary of using resources from cplusplus.com, they have a reputation of having incomplete and inaccurate information throughout their site (can't say anything specifically to the content of the linked page, however).
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      1. USMoses
    2. COYes, the calls should be exactly the same. Also, std::sort should call the predicate at _least_ N * log(N) times, where N is the size of the vector. Eg, for a vector of 1000 elements, it should call the predicate at _least_ 10000 times.
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      1. USMooing Duck
    3. COWhen you write that "`Item` is a simple base class", do you mean to say that some elements of your vector will be instances of subclasses of `Item`? If that's the case, then you'll need your vector to hold pointers to the instances, rather than the instances themselves; `vector<const Item *>` rather than `vector<Item>`.
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      1. USruakh
 

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