Re: constructor and reset method

James Kanze <>
Wed, 27 Feb 2008 07:41:30 -0800 (PST)
On Feb 27, 6:22 am, ""
<> wrote:

On Feb 26, 11:12 pm, Jeff Schwab <> wrote:

Alf P. Steinbach wrote:

* Christopher:
      reset() { *this = B(); }

I've never seen that before, but it does seem like a neat
idea. :)

It seems like it would be (generally speaking) twice as slow
as manual member-by-member initialization, because of (a)
the temporary object's constructor, plus (b) the assignment

If it makes a measurable difference, make the functions inline,
and let the compiler's optimizer take care of it.

Just now I managed to get this working using placement new. Is
there anything bad that can happen with the following code
(assuming the constructor does not throw any exceptions):

class B {
  B () { }
  ~B () { }

  void Reset () {
    new (this) B();

Well assuming the constructor doesn't throw any exceptions, and
assuming that no one ever derives from the class, and probably
assuming one or two other things, it's OK.

In general, there are just too many assumptions in there for me
to ever want to see it in production code.

I've done a good amount of testing on that just now and it
seems to work fine (even when B has other complex object
members, etc.).

Try adding a virtual member (say a virtual destructor), then
deriving. I think you'll find some mighty strange behavior.
And of course, it's very, very hard to guarantee that a
constructor doesn't throw.

James Kanze (GABI Software)
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