Re: + or += (was: why not use i++ instead of ++i in for()?)

From: Hendrik Schober (SpamTrap_at_gmx.de)
Date: 02/24/04

  • Next message: Bill Gates: "How to know the property of constructor function of a class?" 
    Date: Tue, 24 Feb 2004 20:03:16 +0100

    Bo Persson <bop@gmb.dk> wrote:
    > [...]
    >
    > > > > > > s = a + b + c + d;
    >
    > [...]
    >
    > > > > > > s = a;
    > > > > > > s += b;
    > > > > > > s += c;
    > > > > > > s += d;
    >
    > [...]
    >
    > > T operator+(const T& l, const T& r)
    > > {
    > > T tmp(l);
    > > tmp += r; // invoking 'operator+=()' here
    > > return tmp;
    > > }
    > >
    > > This makes it obvious that, regarding
    > > performance, 'operator+()' can never be
    > > better than 'operator+=()', but it will
    > > often be worse.
    >
    > If you are unlucky, this will allocate a buffer and copy l once, then
    > reallocate the buffer and copy l again, before adding r.

      And then it copies again, upon return,
      possibly also re-allocating again.

    > Another implementation looks like this:
    >
    > T operator+(const T& l, const T& r)
    > {
    > T tmp;
    > tmp.reserve(l.size() + r.size());
    >
    > return tmp.append(l).append(r);
    > }
    >
    > A maximum of one memory allocation. Copies each character exactly once.

      Twice. You keep forgetting the return
      value.
      (As a side note, I once had to learn
      that returning the expression might
      confuse the optimizer so that it cannot
      apply the RVO.)

    > [...]
    > > Still, this creates three temporaries,
    > > where Mark's code creates none. And his
    > > code will not trigger any allocations
    > > either.
    > > What is your point?
    >
    > My point is:
    >
    > Best case, three temporaries on stack, but no allocations.

      Plus one assignment.

    > Mark's best case, no temporaries, but s is updated 3 more times.

      3 times plus one assignment, minus three
      temporaries.

    > Worst case, three temporaries allocated, s is reallocated on assignment.
    > Four in total.
    >
    > Mark's worst case, s is reallocated on assignment, and for each append. Four
    > in total.

      Right, in worst case it is as bad as the
      other. Otherwise it's better.

    > s = a + b + c + d;
    >
    > Looks much better. :-)

      Maybe. I use it whenever perfromance is
      not an issue. Generally, this should be
      most of the time. In my case, it is never
      more than 50%. (I usually don't do GUIs.)

    > Bo Persson

      Schobi 

    -- 
SpamTrap@gmx.de is never read
I'm Schobi at suespammers dot org
"Sometimes compilers are so much more reasonable than people."
  Scott Meyers
  • Next message: Bill Gates: "How to know the property of constructor function of a class?"

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