Re: [boost] xml?

Subject: Re: [boost] xml?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2010-04-09 13:50:34

On 04/09/10 11:46, 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.  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.

However if the operation to be performed is implemented as a virtual
function, then no dynamic_casting is needed.  Just call the virtual
function.  I thought this was one of the main selling points
of virtual functions vs. using a switch statement to cover
all elements in a type hierarchy and then performing
the operation at the appropriate case clause.