Subj : Re: OO compilers and efficiency
To   : comp.programming
From : sebasttj
Date : Fri Jul 22 2005 07:05 am



websnarf@gmail.com wrote:
> Here's a trick I'd like to see "improved C++ code" for (requires
> bstrlib, which can be found at http://bstring.sf.net/):
>
>    /* The C++ way */
>    a = CBString ("end");
>    for (i=0; i < 100000; i++) {
>        a = CBString (&tbstrTable[i%8]) + a;
>    }

a = CBString("end");
for(int i=0; i!=100000; ++i) {
  CBString t(&tbstrTable[i%8]);
  t += a;
  swap(t, a);
}

And code a swap function for CBString along the lines of the one for
the STL containers (ie, O(1) time).

> But the reason that C is superior here is that it doesn't rely on
> implicit storage for the base of the object.  The "a = b" line, just
> does a pointer assignment, which essentially just swaps the object
> base.

The reason your C++ version sucked is because you didn't provide and
use a swap function (which, as you said in the quote, is the operation
you wanted). You were using value-semantics assignment as a
replacement, but it does a *lot* more work than you needed.

> A similar case is for block initialization.  STL's vectors, for
> example, requires individual constructing of each element as the vector
> grow, including empty constructors for "anticipated elements".

No, they don't. Memory is allocated (usually using operator new, the
C++ equivalent of malloc -- no constructors are called) in anticipation
of future elements, but they aren't constructed (using placement new)
until they're actually needed.

>  A
> comparable implementation in C can just malloc big blocks of pointers
> and set them all to NULL in a single for-loop or memset or whatever.

Then the C++ version is more efficient, because it doesn't need to do
the memset. :-)

> This also applies to the creation of complicated data structures, say,
> like tries.  In C you can make "custom allocators" which will allocate
> many nodes at once in a single malloc

Wow... I wish someone on the C++ standardization committee had thought
of custom allocators. /sarcasm

> Perhaps there is something fundamental about C++, or OO in general that
> I am not understanding, but how does C++ address this?

It looks to me like you just don't know C++ very well. Most people
don't. (Not that I do, either.)

Josh

.