Re: constructor and reset method

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Wed, 27 Feb 2008 11:48:41 +0100

Message-ID:

<13sag0eairkarb8@corp.supernews.com>

* Juha Nieminen:

Alf P. Steinbach wrote:

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

  Btw, a bit related to that, can anyone spot some problem with this:

A::A(const A& rhs)
{
    *this = rhs;
}

A& operator=(const A& rhs)
{
  // Tons of member variable assignments here
}

  The idea would be, of course, to save writing all the member variable
initializations twice.

It is again a problem with derived classes, but here "only" with efficiency and
expectations of maintainance programmers (the latter the most serious).

Another problem is that it requires data members to be default-constructible.

(I really wish there was a way to call the compiler-generated "default
copy constructor" and the "default assignment operator" from your own
versions, so that you only had to concentrate on those members for which
the default assignment is not enough. The problem with having to
construct/assing every single member manually is that if you add a new
member variable in the future you have to remember to add it to both the
copy constructor and assignment operator. If you forget, the compiler
won't give you even a warning, and the program may malfunction in
strange ways, which will sometimes be very hard to debug.)

That's what the swap idiom for assignment is all about (in addition to exception
safety).

Cheers, & hth.,

- Alf

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