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PODo mixins solve fragile base class issues?
 

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  1. PODo mixins solve fragile base class issues?
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    <p>In a class on programming languages, my professor quotes mixins as one of the solutions to the fragile base class problem. Wikipedia also used to list (Ruby) mixins as a solution for the fragile base class problem, but some time ago <a href="http://en.wikipedia.org/w/index.php?title=Fragile_base_class&amp;diff=451336813&amp;oldid=446645464" rel="nofollow">someone removed</a> the reference to mixins. I still suspect that they might somehow have an advantage over inheritance with regards to the fragile base class problem. Otherwise, why would a professor say that they help?</p> <p>I'll give an example of a possible problem. This is a simple Scala implementation of the (Java) problem that the professor gave us to illustrate the problem.</p> <p>Consider the following base class. Assume that this is some very efficient special implementation of a list, and that more operations are defined on it.</p> <pre class="lang-scala prettyprint-override"><code>class MyList[T] { private var list : List[T] = List.empty def add(el:T) = { list = el::list } def addAll(toAdd:List[T]) : Unit = { if (!toAdd.isEmpty) { add(toAdd.head) addAll(toAdd.tail) } } } </code></pre> <p>Also consider the following trait, which is supposed to add <code>size</code> to the list above.</p> <pre class="lang-scala prettyprint-override"><code>trait CountingSet[T] extends MyList[T] { private var count : Int = 0; override def add(el:T) = { count = count + 1 super.add(el) } override def addAll(toAdd:List[T]) = { count = count + toAdd.size; super.addAll(toAdd); } def size : Int = { count } } </code></pre> <p>The problem is that whether or not the trait will work depends on how we implement <code>addAll</code> in the base class, i.e. the functionality provided by the base class is 'fragile', just as would be the case with a regular <code>extends</code> in Java or any other programming language.</p> <p>For example, if we run the following code with <code>MyList</code> and <code>CountingSet</code> as defined above, we get back <code>5</code>, whereas we would expect to get <code>2</code>.</p> <pre class="lang-scala prettyprint-override"><code>object Main { def main(args:Array[String]) : Unit = { val myCountingSet = new MyList[Int] with CountingSet[Int] myCountingSet.addAll(List(1,2)) println(myCountingSet.size) // Prints 5 } } </code></pre> <p>If we change <code>addAll</code> in the base class (!) as follows, the trait <code>CountingSet</code> works as expected.</p> <pre class="lang-scala prettyprint-override"><code>class MyList[T] { private var list : List[T] = List.empty def add(el:T) = { list = el::list } def addAll(toAdd:List[T]) : Unit = { var t = toAdd; while(!t.isEmpty) { list = t.head::list t = t.tail } } } </code></pre> <p>Keep in mind that I am anything but a Scala expert!</p>
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    1. COWell maybe it's just me but I don't see how traits/mixins solve fragile base class issues. They have many other qualities though.
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    2. COI do not see it either, so it's not just you :). If they're indeed of no help, it might be really difficult to answer my question... I am willing to accept a 'your professor is wrong' solution as long as it is convincing. Maybe it has something to do with traits not being 'real mixins'?
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