Re: Some misc C++ questions (multimap, derived class function argument,
virtual static)
* 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
{
public:
virtual void combine(const Base &other, Base &result) = 0;
}
class Derived : public Base
{
private:
string _name;
public:
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
ones.
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?
Like
template< class T >
T* createRandomInstance() { return new T(); }
class Derived: public Base
{
public:
Derived( char const [] ) {}
};
template<>
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