Re: Avoid automatic copy constructor generation

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Tue, 28 Oct 2008 09:19:46 -0700 (PDT)
Message-ID:
<47b02f9c-9b29-4a75-9f7e-977432c5ea11@f40g2000pri.googlegroups.com>
On Oct 27, 11:03 pm, Jeff Schwab <j...@schwabcenter.com> wrote:

Ian Collins wrote:

Erik Wikstr=F6m wrote:

On 2008-10-27 19:29, Jeff Schwab wrote:

Juha Nieminen wrote:

Ian Collins wrote:

I can see the benefit of "= default", but what benefits does "==

 delete"

offer over private constructors?

  Declaring a constructor private is more a hack than a clean idiom.=

 If

you accidentally call the copy constructor from the class itself, yo=

u

will not get a clean compiler error, but an obscure linker error, wh=

ich

is not really the best possible solution.

That's true if the private constructors are declared directly in the
non-copyable class. I wonder whether =delete offers anything over
EBO/inheritance-based utilities like boost::noncopyable.

Elegance and simplicity. A private copy-constructor is a hack but
boost::noncopyable is much worse, a complex solution to a simple
problem. Why should I have to inherit from some other class just to ma=

ke

my class non-copyable?


It may be a hack, but it's an idiomatic one that's been used
since the epoch of C++.

I don't consider it to be a hack, but an aid to
documentation. The reader can tell at a glance that the
class isn't copyable. It also saves typing!


Agreed. It's self-documenting, and doesn't require the reader
to be familiar with yet another language feature.


Yes and no. I used the techique back in the early 1990's. I
dropped it for awhile because readers weren't familiar with it.
The private copy constructor was the standard idiom, which they
immediately recognized, where as they didn't know what
UncopiableObject did; at the time, at least, it didn't occur to
them that a base class might have not direct interface, and they
expected something more. Thanks to things like std::iterator<>,
std::unary_function<>, etc., modern C++ programmers are far more
open about this, and when they see a class called
UncopiableObject, they don't expect anything more than that it
can't be copied. Some I've reverted to doing things the way I
did 25 years ago.

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