Re: Variables in for loop (style issue)

Thorsten Ottosen <>
28 May 2006 07:17:14 -0400
Anders J. Munch wrote:

Thorsten Ottosen wrote:
  > Anders J. Munch wrote:
  >> It was your example. My rewrite has the same functionality. If
  >> you think there are additional mutating operations that need to be
  >> accounted for, then you should have included them in your
  >> challenge.
  > <g>
  > Please consider them included in the challenge.

Details, please! Don't ask me to come up with a couple of mutating
operations that, without const, can't be implemented without giving up
on either encapsulation or performance. The problem is I don't
believe they exist - if you do, then you should provide the specifics.

It's fairly easy to see they do exist: just imagine a container
operations for which it is cheaper to modify one element than to
copy the whole object with one element replaced.

All I can do is show a random assortment of mutating operations that
most definitely _can_ be implemented. Bar is still immutable, of
course, but I'll have Foo grow a few methods that correspond to
changing the vector<int> in the original design:

class Foo
      Bar m_bar;
      Bar bar() const;

      // Make bar() return bar_
      void replace_all_of_bar(Bar bar_);

      // Increase each element returned by bar() by one.
      void inc_bar();

      // Make bar()[index] return new_value.
      void replace_bar_component(size_t index, int new_value);

Performance characteristics are different, when compared to the
original with a vector<int> member returned by const reference. Not
worse, not better, just different.

there's only three ways to characterize performance: either it is
slower, just as fast or faster. What you are saying is that
there's a fourth way: they have the same performance, but are different
anyway. What's the logic in that?

replace_bar_component has gone from O(1) to O(bar().size()) in the
straightforward implementation, which is bad.

That is pretty bad.

But, barring
copy-construction costs, replace_all_of_bar has gone from
O(bar().size()) to O(1), which is good.

It's still an O(bar().size()) operation.

For some types, copy-construction is faster or just as
fast as assigning a reference.

And overall performance
should be just fine, as long as I stick with high-level operations
such as inc_bar, instead of fiddling with individual low-level
elements using replace_bar_component.

What makes you think that is possible? A generic container cannot
assume very much about the elements it stores, and certainly not that an
element will be incrementable.


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