Re: speed of Obj-C vs. C++(slightly OT)

[Graham Lee]

Jim McLoughlin wrote:
> Hi Drik
>
> On Tuesday, April 22, 2003, at 01:36 AM, Dirk Theisen wrote:
>
> > You can always include C libraries for maximum speed.
> > AFAIK, some computational finance libs are in plain C anyway so you 
> > can get by with using C for the computations and ObjC for the rest 
> > (where message passing times do not matter).
>
>
> Thanks for the response.  I realized after posting that it all depends 
> on how you split things up between C/C++ or C/Obj-C.
>
> I guess my question is that, assuming these ratios are equal, which is 
> faster.

C++.  The reason is that C++ is *not* an Object-Oriented language, it 
has structs and it has functions.  When you have a method in C++ (such 
as myClass.doSomething(withThis);), the method is resolved at *compile 
time* into a standard ol' C function call, which probably takes all of 
around four cycles for a function that takes no arguments, and uses 
about the same number to leave if the function returns void.
Objective-C, on the other hand, does support OOP, including dynamic 
message lookup at runtime.  This is why, in Objective-C, it is possible 
to change the target of a message while the program is running.  This 
then requires a longer time to look up messages, in that they actually 
have to be looked up.  I *think* this takes around 25 cycles per message 
call (a test I did with the GNU runtime on i686 suggests this).

It is possible to speed this up slightly, by caching message calls or 
even by calling objc_msgsend() directly.  However once you do this, 
you'll probably find it easier and just as fast to write your code in 
C++ or straight C.  Although what's a couple of dozen CPU cycles every 
now and then?  The only situation you might find it to be a bind would 
be in highly recursive code.

--
Graham Lee, Wadham College, OX1 3PN