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

From:

Dave Abrahams <dave@boostpro.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 31 Oct 2011 23:12:09 -0700 (PDT)

Message-ID:

<m2ipn557af.fsf@boostpro.com>

on Sun Oct 30 2011, Ralph Zhang <ralph.j.zhang-AT-gmail.com> 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)

#include <functional>
#include <iostream>

struct S
{
  S() = default;
  S(S const&) = delete;
  S& operator=(S const&) = delete;
  S(S&&) { }
  void operator()() { }
};

std::function<void()> func;

void make_func()
{
  S s;
  func = std::bind(std::move(s)); // This won't compile
}

int main()
{
  make_func();
  func();
}

You can't do it directly. std::function's job is to hide from the type
system all details about the object it's storing, e.g. how it is called,
destroyed, and copied. That includes the detail of whether or not the
stored object *can* be copied. It works by generating functions to
call/copy/delete the stored object at the point where the object is
stored, and later calling those functions indirectly through function
pointers (or vtables; same thing). If those functions can't compile
(say because the stored object is noncopyable), compilation fails.

You have two alternatives:

1. Build what is essentially a std::move_function, a move-only function
   wrapper, and use that.

2. Build a copyable wrapper around a shared_ptr to your move-only
   function object, and store that wrapper in std::function. Of course
   that means if your underlying function object has any mutable state,
   the resulting objects don't have value semantics. Considering that
   your operator() is non-const, that could be a worry.

By the way, a shorter and equally-valid test would be simply:

   std::function<void()> func = S();

Cheers,

--
Dave Abrahams
BoostPro Computing
http://www.boostpro.com

      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
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