[xsd-users] Substitution groups & determining run-time type

[Boris Kolpackov]

Hi Ian,

Durkan, Ian <ian.durkan at progeny.net> writes:

> I'm in a situation where I need to determine the type of an element at
> runtime, when the element may be any of a substitution group.  I
> understand how to use dynamic_cast<T> to determine the run-time type of
> the element object, but I'm concerned about the performance of this
> method vs. accessing the DOMElement behind the object and calling its
> getTagName function and performing a string comparison.  Does anyone
> have information about which approach would yield better performance?  

I am pretty sure dynamic_cast is going to be faster. I made a little
test which I compiled with g++ 4.3.4 with -O3 and ran on 2.2Ghz Nahalem
Xeon (E5520). Here are the results:

virtual call : 0.0026 microseconds/call
dynamic cast : 0.0104 microseconds/call
strcmp       : 0.0244 microseconds/call

So dynamic_cast (from base to derived) is about 4 times as expensive 
as a virtual function call. And string comparison is about 2 times as
expensive as dynamic_cast. This is not taking into account the time
it will take to get the string from DOMElement (an additional virtual
function call).

You may also want to check Technical Report on C++ Performance[1],
section 5.3.8. In particular, it mentions a C++ compiler implementation
with this dynamic_cast case and a virtual function call taking the
same time. So it also depends on the compiler.

But, overall, I think the times for all three cases are negligible
on modern hardware. I had to run the test loop 100,000,000 times --
anything less was rounded to 0.

The two test files are attached. If you want to run the test, check the
assembly code first to make sure your C++ compiler didn't optimize 
anything important away.

[1] http://www.open-std.org/jtc1/sc22/wg21/docs/TR18015.pdf

Boris