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Subject: Re: [boost] [core/noncopyable][test/boost::unit_test::singleton] massive test failures
From: Peter Dimov (lists_at_[hidden])
Date: 2014-08-21 09:08:40

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. :-)

constexpr on the constructor enables noncopyable, and its descendants, to be
statically initialized. I suppose this makes sense; a mutex, for example, is
noncopyable but it may be desirable for it to support static initialization.

=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.

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. :-)

(There is a core defect report against that though, if I'm not mistaken.)

In summary, I don't see the new noncopyable as better than the old one in
any significant way.

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