On 04/09/2010 12:46 PM, Stewart, Robert wrote:
Stefan Seefeld wrote:
On 04/09/2010 12:34 PM, Larry Evans wrote:
On 04/09/10 10:48, Mathias Gaunard wrote:
Some could argue that the point of a base class is moot if you have to downcast it to make anything useful with it,
But boost.variant has to do the equivalent of downcasting based on discriminant (the value returned by which()).
No, accessing a member of a union is not really a cast, neither conceptually nor technically.
Conceptually, they are doing the same: determining whether the requested type is appropriate and returning the appropriately typed value.
Right, but that's not the same as casting (in the sense of narrowing down a more generic to a more specific type). The difference is that in one case both types have a common ancestor / super-type (implying a common interface of some form), while in the other case they don't.
However, dynamic_cast does so by comparing RTTI objects, which may be more expensive than the discriminator comparison, and then by apply fixups to the this pointer to account for MI, virtual bases, etc.
What's more, once the dynamic_cast has been done, the various functions calls may still wind up being virtual, depending upon how things are implemented. When using Boost.Variant, the types used could have no virtual functions.
I'm not sure why you are saying all this. The point is really that there is no common interface that integral / numerical types share with nodes and node sets. Thus there is no sense in having these share some common interface that would justify a common base class, making a union-like accessor (such as boost.variant) the obvious accessor for XPath query return values. Stefan -- ...ich hab' noch einen Koffer in Berlin...