Talk:Trait

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Comments on Trait <comments />

Miles Elam said ...

I am primarily interested in properly typing the multiple class encapsulation case. E.g. having prev/next field in the item class and manipulating them in only by the list. Moreover I'd like to write this (and type this) in a generic way. Looks like it will be possible to do it in C++, but the solution will definitely not be like in Java/Scala - rather upside-down...

--JaroslavTulach 22:25, 5 September 2012 (UTC)

jtulach said ...

class ll_item;
 
class llist {
  public:
    void add(ll_item& p);
    void remove(ll_item& p);
 
    llist(void);
   ~llist();
};
 
class ll_item {
    friend llist;
  private:
    ll_item* next;
    ll_item* prev;
};
 
 
class person_item : public ll_item {
  protected:
    int age;
    const char* name;
 
  public:
    person_item(int age, const char* name);
};
 
class animal_item : public ll_item {
  protected:
    const char* name;
 
  public:
    animal_item(const char* name);
};
 
int main(int argc, char* argv[])
{
  person_item a(10, "Ben");
  person_item b(20, "Nora");
  person_item c(30, "John");
 
  animal_item x("Fifi");
  animal_item y("Bobika");
  animal_item z("Bill");
 
  llist l;
  l.add(b);
  l.add(c);
  l.add(a);
  l.add(x);
  l.add(y);
  l.add(z);
}

Right, now the question is how to generify this (as the example above does not feel type safe enough) - e.g. how to turn into a C++ template?

I don't want to have list of items, but rather list of persons and another list of animals. When I ask for an item from each list, I would expect I get a person in the former case and an animal in the latter. Right now I just get a ll_item. That is efficient, but not really typesafe.

I have an unfinished prototype with template and compared to Java or Scala it feels a bit upside down. Interesting clash of cultures.

--JaroslavTulach 08:26, 12 September 2012 (UTC)

jtulach said ...

class persons_list : public llist {
  public:
    person_item *get(int idx)
    {
      return dynamic_cast<person_item *>(llist::get(idx));
    }
};

The question Why this is not type-safe? may reveal the progress OOP made during last twenty years. In the middle of nineties Java was claimed a safe language as it could not cause segmentation faults. The C++'s dynamic_cast is a safety of similar kind. It is a runtime check. It is not bad thing, but as I am coding in Java most of the time, I was not calling for something as basic.

When I mentioned type safety, I meant compile type safety. The above dynamic cast does not guarantee that I cannot add animal_item into the llist and try to obtain person_item. To achieve such kind of safety one needs a form of algebraic types (nicely illustrated in the revolutionary essay). Both Scala and Java (since version 1.5) provide a way to marry algebraic types with OOP inheritance and thus provide compile time safety (as code snippets for the direct linked list show).

--JaroslavTulach 07:42, 17 September 2012 (UTC)