Re: Storing noncopyable but movable object in std::function

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Mon, 31 Oct 2011 23:11:23 -0700 (PDT)
On 2011-10-31 06:43, Ralph Zhang wrote:

Suppose I have a functor s, which is noncopyable but movable, how can
I store it in a std::function? i.e, how to make the following code
compile? (using gcc 4.6)

First of all, you don't need std::bind in your example, the assignment
of a callable functor to std::function can be done directly as

 func = std::move(s);

This doesn't solve your problem though. There exists a fundamental
requirement, that the wrapped functor is CopyConstructible. This is
related to the fact, that std::function performs type-erasure on the
wrapped function object. So, if you need to wrap your move-only functor
anyway, you either need to wrap a reference of it (e.g. using
std::reference_wrapper) or you need to perform dynamic memory allocation
and user a wrapping function object of your move-only functor e.g.

struct WS
  std::shared_ptr<S> ps;
  void operator()() { (*ps)(); }

It depends on the life-cycle of the std::function wrapper, whether you
can use the reference_wrapper approach (Obviously you cannot return an
automatic variable of S out of the function scope), or whether you need
to use dynamic memory allocation.

HTH & Greetings from Bremen,

Daniel Kr?gler

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