Is copying automatically prohibited in classes derived from classed derived from Boost noncopyable?

Question

For example:

class Foo : boost::noncopyable
{
    // ...
};

class Bar : public Foo
{
    // ...
};

Is Bar non-copyable?

+1 because this is a very well-formed question. That I think the answer is a bit obvious is not relevant. :) — Lightness Races in Orbit, Jul 28 '11 at 18:34
I had to think about it for a bit before it became apparent. I was surprised that no one ever asked this question. — Emile Cormier, Jul 28 '11 at 18:38

score 8 · Accepted Answer · edited May 11 '15 at 18:18

8

By default it is non-copyable, unless you create a custom copy-constructor and avoid calling a base copy-constructor there.

See also Explicitly-defaulted and deleted special member functions introduced in C++11. Even though making a copy constructor/operator private solves the problem, the compiler generates a diagnostic message that is far from pretty and obvious, so deleted copy constructors/operators are there in C++11 to solve this problem.

edited May 11 '15 at 18:18

NHDaly

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answered Jul 28 '11 at 18:32

1

..and that would almost certainly be "broken". – Lightness Races in Orbit Jul 28 '11 at 18:34
@Tomalak: Yes :-) But still possible, hehe. – Jul 28 '11 at 18:36

Nawaz · Answer 2 · 2011-07-28T18:42:19.090

2

Assuming the derived class doesn't have custom copy-constructor which avoids calling the noncopyable copy-constructor, then yes. At all level, all derived classes of boost::noncopyable would be non-copyable. As object of derived class also contains the subobject of boost::noncopyable which is non-copyable, that means no derived class can be copyable without base-class being copyable,

edited Jul 28 '11 at 18:42

answered Jul 28 '11 at 18:32

Nawaz

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score 1 · Answer 3 · answered Jul 28 '11 at 18:33

1

Bar derives from boost::noncopyable (even though it's not a direct inheritance), so yes.

answered Jul 28 '11 at 18:33

Lightness Races in Orbit

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ks1322 · Answer 4 · 2011-07-28T18:54:09.273

1

Yes, if it were copyable then all base classes must be copyable, but boost::noncopyable is non-copyable

edited Jul 28 '11 at 18:54

answered Jul 28 '11 at 18:34

ks1322

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"if it [were] copyable then all derived classes must be copyable" <-- not so. Consider `struct Bar : struct Foo, boost::noncopyable {};`. `Foo` is copyable (say), but `Bar` is not. – Lightness Races in Orbit Jul 28 '11 at 18:35
1

@Tomalak: Yes, I've mixed up `base` and `derived` - fixed now – ks1322 Jul 28 '11 at 18:54

Is copying automatically prohibited in classes derived from classed derived from Boost noncopyable?

4 Answers4