From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2007-09-21 12:25:37
Igor.Smirnov_at_[hidden] wrote:
> May be I am behind progress, but I do not know a way to make cloning
> pointer (or that Mathias' value object, but this seems to be the same in
> principle) to be non-intrusive. You anyway need virtual cloning function
> in each class of the hierarchy.
Boost.Any, for example, copies dynamically objects without having to
modify their type.
Here is a simplistic possible implementation for our case.
template<typename T, typename D>
T* clone_function(const T& t)
{
return new D(static_cast<const D&>(t));
}
template<typename T>
class Container
{
template<typename D>
Container(const D& d) : obj(new D(d)), clone(clone_function<T, D>)
{
}
// copy constructor, operator=, etc.
~Container()
{
delete obj;
}
T* obj;
T* (*clone)(const T&);
};
As I said earlier, this has a few flaws:
- one word per object is wasted. Polymorphic objects already have one
word that identifies their type (the pointer to the vtable)
- it assumes that the static type of the object the container acquires
is the same as its dynamic type.