Re: constructor and reset method
On Feb 27, 11:48 am, "Alf P. Steinbach" <al...@start.no> wrote:
* 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).
I don't think it's really a problem with derived classes,
because at this point, they don't exist yet. (Their constructor
hasn't yet been called.) However...
Another problem is that it requires data members to be
default-constructible.
Worse: if some of the data has do nothing constructors, and the
operator= expects it to be initialized. Pointers to dynamic
memory would be a classical case---if the operator= results in
the old values being passed to delete, delete's not going to
like getting passed some random bits.
As you say...
(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).
In simple cases, you may be able to do even simpler than the
swap idiom, but it is the guaranteed general solution (provided
all of the members support it as well).
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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