Move-only version of std::function
No, there is no move-only version of std::function in the C++ std library. (As of C++14)
Fastest possible delegates is an implementation of a std::function like class that happens to be faster than most std::function implementations in many std libraries, and it should be easy to fork into a move and copy version.
Wrapping your move only function object into a shared_ptr<F> in a class with a forwarding operator() is another approach.
Here is a task sketch:
template<class Sig>
struct task;
namespace details {
template<class Sig>
struct task_iimpl;
template<class R, class...Args>
struct task_iimpl<R(Args...)> {
virtual ~task_iimpl() {}
virtual R invoke(Args&&...args) const = 0;
};
template<class F, class Sig>
struct task_impl;
template<class F, class R, class...Args>
struct task_impl<F,R(Args...)>:
task_iimpl<R(Args...)>
{
F f;
template<class T>
task_impl(T&& t):f(std::forward<T>(t)) {}
virtual R invoke(Args&&...args) const override {
return f( std::forward<Args>(args...) );
}
};
template<class F, class...Args>
struct task_impl<F,void(Args...)>:
task_iimpl<void(Args...)>
{
F f;
template<class T>
task_impl(T&& t):f(std::forward<T>(t)) {}
virtual void invoke(Args&&...args) const override {
f( std::forward<Args>(args...) );
}
};
}
template<class R, class...Args>
struct task<R(Args...)> {
virtual ~task_iimpl() {}
R operator()(Args...args) const {
return pImpl->invoke(std::forward<Args>(args...));
}
explicit operator bool()const{ return static_cast<bool>(pImpl); }
task(task &&)=default;
task& operator=(task &&)=default;
task()=default;
// and now for a mess of constructors
// the rule is that a task can be constructed from anything
// callable<R(Args...)>, destroyable, and can be constructed
// from whatever is passed in. The callable feature is tested for
// in addition, if constructed from something convertible to `bool`,
// then if that test fails we construct an empty task. This makes us work
// well with empty std::functions and function pointers and other tasks
// that are call-compatible, but not exactly the same:
struct from_func_t {};
template<class F,
class dF=std::decay_t<F>,
class=std::enable_if_t<!std::is_same<dF, task>{}>,
class FR=decltype(std::declval<F const&>()(std::declval<Args>()...)),
std::enable_if_t<std::is_same<R, void>{} || std::is_convertible<FR, R>{} >*=0,
std::enable_if_t<std::is_convertible<dF, bool>{}>*=0
>
task(F&& f):
task(
static_cast<bool>(f)?
task( from_func_t{}, std::forward<F>(f) ):
task()
)
{}
template<class F,
class dF=std::decay_t<F>,
class=std::enable_if_t<!std::is_same<dF, task>{}>,
class FR=decltype(std::declval<F const&>()(std::declval<Args>()...)),
std::enable_if_t<std::is_same<R, void>{} || std::is_convertible<FR, R>{} >*=0,
std::enable_if_t<!std::is_convertible<dF, bool>{}>*=0
>
task(F&& f):
task( from_func_t{}, std::forward<F>(f) )
{}
task(std::nullptr_t):task() {}
// overload resolution helper when signatures match exactly:
task( R(*pf)(Args...) ):
task( pf?task( from_func_t{}, pf ):task() )
{}
private:
template<class F,
class dF=std::decay_t<F>
>
task(from_func_t, F&& f):
pImpl( std::make_unique<details::task_impl<dF,R(Args...)>>(
std::forward<F>(f)
)
{}
std::unique_ptr<details::task_iimpl<R(Args...)> pImpl;
};
but it has not been tested or compiled, I just wrote it.
A more industrial strength version would include a small buffer optimization (SBO) to store small callables (assuming they are movable; if not movable, store on heap to allow moving), and a get-pointer-if-you-guess-the-type-right (like std::function).
As others have pointed out, there is no move-only version of std::function in the library. Following is a work-around that the reuses (abuses?) std::function and allows it to accept move-only types. It is largely inspired by dyp's implementation in the comments, so a lot of the credit goes to him:
#include <functional>
#include <iostream>
#include <type_traits>
#include <utility>
template<typename T>
class unique_function : public std::function<T>
{
template<typename Fn, typename En = void>
struct wrapper;
// specialization for CopyConstructible Fn
template<typename Fn>
struct wrapper<Fn, std::enable_if_t< std::is_copy_constructible<Fn>::value >>
{
Fn fn;
template<typename... Args>
auto operator()(Args&&... args) { return fn(std::forward<Args>(args)...); }
};
// specialization for MoveConstructible-only Fn
template<typename Fn>
struct wrapper<Fn, std::enable_if_t< !std::is_copy_constructible<Fn>::value
&& std::is_move_constructible<Fn>::value >>
{
Fn fn;
wrapper(Fn&& fn) : fn(std::forward<Fn>(fn)) { }
wrapper(wrapper&&) = default;
wrapper& operator=(wrapper&&) = default;
// these two functions are instantiated by std::function
// and are never called
wrapper(const wrapper& rhs) : fn(const_cast<Fn&&>(rhs.fn)) { throw 0; } // hack to initialize fn for non-DefaultContructible types
wrapper& operator=(wrapper&) { throw 0; }
template<typename... Args>
auto operator()(Args&&... args) { return fn(std::forward<Args>(args)...); }
};
using base = std::function<T>;
public:
unique_function() noexcept = default;
unique_function(std::nullptr_t) noexcept : base(nullptr) { }
template<typename Fn>
unique_function(Fn&& f) : base(wrapper<Fn>{ std::forward<Fn>(f) }) { }
unique_function(unique_function&&) = default;
unique_function& operator=(unique_function&&) = default;
unique_function& operator=(std::nullptr_t) { base::operator=(nullptr); return *this; }
template<typename Fn>
unique_function& operator=(Fn&& f)
{ base::operator=(wrapper<Fn>{ std::forward<Fn>(f) }); return *this; }
using base::operator();
};
using std::cout; using std::endl;
struct move_only
{
move_only(std::size_t) { }
move_only(move_only&&) = default;
move_only& operator=(move_only&&) = default;
move_only(move_only const&) = delete;
move_only& operator=(move_only const&) = delete;
void operator()() { cout << "move_only" << endl; }
};
int main()
{
using fn = unique_function<void()>;
fn f0;
fn f1 { nullptr };
fn f2 { [](){ cout << "f2" << endl; } }; f2();
fn f3 { move_only(42) }; f3();
fn f4 { std::move(f2) }; f4();
f0 = std::move(f3); f0();
f0 = nullptr;
f2 = [](){ cout << "new f2" << endl; }; f2();
f3 = move_only(69); f3();
return 0;
}
Working version to coliru.
Yes, there is a proposal for std::move_only_function in the current draft of C++23, adopted 2021-10:
This paper proposes a conservative, move-only equivalent of
std::function.
See also the cppreference entry on std::move_only_function:
Class template std::move_only_function is a general-purpose polymorphic function wrapper. std::move_only_function objects can store and invoke any constructible (not required to be move constructible) Callable target -- functions, lambda expressions, bind expressions, or other function objects, as well as pointers to member functions and pointers to member objects.
...
std::move_only_function satisfies the requirements of MoveConstructible and MoveAssignable, but does not satisfy CopyConstructible or CopyAssignable.