Re: Some misc C++ questions (multimap, derived class function argument, virtual static)

"Alf P. Steinbach" <>
Sun, 13 Sep 2009 21:38:24 +0200
* Digital Puer:

I have several C++ questions:

1. Why are the interfaces for multiset and multimap so complicated for
getting multiple values?

Instead of
multimap<Key, Value>

I usually prefer
map<Key, vector<Value> >

Isn't that much easier to use?

No opinion.

2. Suppose I have a base class that looks like:

class Base
        virtual void combine(const Base &other, Base &result) = 0;

class Derived : public Base
        string _name;
        void combine(const Derived &other, Derived &other) {}

When I try to instantiate Derived, g++ is telling me that the
pure virtual method Base.combine() is not implemented.
Why does Derived.combine() above not override Base.combine()?

Read up on the Liskov Substitution Principle.

Essentially if you could override that way you could then do

    Derived d;
    Base b;
    Base& r = d;

    r.combine( b, b );

where the Derived object gets Base objects in as arguments, but expects Derived

Instead, I have to do some casting, like:

  void combine(const Base &other, Base &result)
        Derived &a = (Derived &)other;
        string combination = _name + a._name;

        ((Derived &)result)._name = combination;

Is there a better approach?

Don't cast.

E.g. you might add virtual member routines that give access to the name.

3. For my abstract Base class, I want to specify a static "Factory"
method that produces a Base*:

class Base
   virtual static Base* createRandomInstance();

I want to force all my Derived classes to provide such a factory
method that produces a Derived *.

However, apparently I cannot have a virtual static method
in C++ (or in Java). What is the best way to force that all
derived classes have such a factory method?

Why not implement a generic factory that works for nearly all derived classes?


   template< class T >
   T* createRandomInstance() { return new T(); }

   class Derived: public Base
       Derived( char const [] ) {}

   Derived* createRandomInstance<Derived>() { return new Derived(""); }

   Base* p = createRandomInstance<Base>();

If the generic version works for a derived class, fine. If not, then using it
for a derived class probably won't compile. So then you've "forced" the derived
class to provide a specialization.

Cheers & hth.,

- Alf

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