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Subject: Re: [boost] [core/noncopyable][test/boost::unit_test::singleton] massive test failures
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-08-21 09:18:49

On Thursday 21 August 2014 16:08:40 Peter Dimov wrote:
> John Maddock wrote:
> > 1) Why on earth is:
> >
> > BOOST_CONSTEXPR noncopyable() = default;
> >
> > Better than
> >
> > noncopyable() {}
> I can tell you what's the difference, but not why it's better. :-)
> =default makes the constructor trivial. A trivial constructor can be
> omitted. It's not clear to me why a noncopyable class would need to have a
> trivial constructor.
> Similarly, =default on the destructor instead of {} makes it trivial. A
> trivial destructor may be omitted. It makes approximately zero sense for a
> noncopyable class to have a trivial destructor.

Why do you think it's not sensible to have a trivial default
constructor/destructor for a non-copyable class? I don't think noncopyable
should guess on user's class semantics. It should do exactly what it is
required to and nothing more, and defaulted functions achieve that.

> The funniest part is that =delete on the copy constructor makes the copy
> constructor trivial as well, in C++11. A trivial copy constructor means that
> the class is copyable with memcpy. :-)

Technically, you can apply memcpy to any object and deal with the
consequences. I don't see why noncopyable should attempt to do anything about
it (and having non-trivial constructor actually doesn't do anything about it).

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