+template<class T> class Task;
+
+/** Type-erased run-once task
+ *
+ * * Like std::function but callable only once.
+ * However, it works with move-only types.
+ *
+ * * Like std::packaged_task<> but without the shared state.
+ */
+template<class R, class... Args>
+class Task<R(Args...)> {
+ // Placeholder for some class type:
+ struct whatever {};
+
+ // Union used for storage:
+ using TaskUnion =
+ typename std::aligned_union_t<0, void*, std::pair<void (*)(), void*>, std::pair<void (whatever::*)(), whatever*>>;
+
+ // Is F suitable for small buffer optimization?
+ template<class F>
+ static constexpr bool canSBO()
+ {
+ return sizeof(F) <= sizeof(TaskUnion) &&
+ alignof(F) <= alignof(TaskUnion);
+ }
+
+ static_assert(canSBO<std::reference_wrapper<whatever>>(),
+ "SBO not working for reference_wrapper");
+
+ // Call (and possibly destroy) the function:
+ using call_function = R (*)(TaskUnion&, Args...);
+ // Destroy the function (of needed):
+ using destroy_function = void (*)(TaskUnion&);
+ // Move the function (otherwise memcpy):
+ using move_function = void (*)(TaskUnion& dest, TaskUnion& src);
+
+ // Vtable of functions for manipulating whatever is in the TaskUnion:
+ struct TaskVtable {
+ call_function call;
+ destroy_function destroy;
+ move_function move;
+ };
+
+ TaskUnion buffer_;
+ const TaskVtable* vtable_ = nullptr;
+
+ void clear()
+ {
+ if (vtable_ && vtable_->destroy)
+ vtable_->destroy(buffer_);
+ }
+
+public:
+ Task() = default;
+ explicit Task(std::nullptr_t) { /* Nothing to do */}
+ ~Task()
+ {
+ this->clear();
+ }
+
+ Task(Task const&) = delete;
+
+ Task(Task&& that) noexcept
+ {
+ if (that.vtable_ && that.vtable_->move)
+ that.vtable_->move(buffer_, that.buffer_);
+ else
+ std::memcpy(&buffer_, &that.buffer_, sizeof(buffer_));
+ vtable_ = std::move(that.vtable_);
+ that.vtable_ = nullptr;
+ }
+ Task& operator=(Task const& that) = delete;
+ Task& operator=(Task&& that) noexcept
+ {
+ this->clear();
+ if (that.vtable_ && that.vtable_->move)
+ that.vtable_->move(buffer_, that.buffer_);
+ else
+ std::memcpy(&buffer_, &that.buffer_, sizeof(buffer_));
+ vtable_ = std::move(that.vtable_);
+ that.vtable_ = nullptr;
+ return *this;
+ }
+
+private:
+ template <class F> typename std::enable_if_t<canSBO<F>()> init(F code)
+ {
+ const static TaskVtable vtable {
+ // Call:
+ [](TaskUnion& buffer, Args... args) {
+ auto* src = reinterpret_cast<F*>(&buffer);
+ F code = std::move(*src);
+ src->~F();
+ // NOTE: std::forward<Args>(args)... is correct.
+ return code(std::forward<Args>(args)...);
+ },
+ // Destroy:
+ std::is_trivially_destructible<F>::value ?
+ static_cast<destroy_function>(nullptr) :
+ [](TaskUnion& buffer) {
+ auto* code = reinterpret_cast<F*>(&buffer);
+ code->~F();
+ },
+ // Move:
+ [](TaskUnion& dst, TaskUnion& src) {
+ auto* src_code = reinterpret_cast<F*>(&src);
+ auto* dst_code = reinterpret_cast<F*>(&dst);
+ new(dst_code) F(std::move(*src_code));
+ src_code->~F();
+ }
+ };
+ new(&buffer_) F(std::move(code));
+ vtable_ = &vtable;
+ }
+
+ template <class F> typename std::enable_if_t<not canSBO<F>()> init(F code)
+ {
+ const static TaskVtable vtable {
+ // Call:
+ [](TaskUnion& buffer, Args... args) {
+ // Delete F when we go out of scope:
+ std::unique_ptr<F> code(*reinterpret_cast<F**>(&buffer));
+ // NOTE: std::forward<Args>(args)... is correct.
+ return (*code)(std::forward<Args>(args)...);
+ },
+ // Destroy:
+ [](TaskUnion& buffer) {
+ F* code = *reinterpret_cast<F**>(&buffer);
+ delete code;
+ },
+ // Move:
+ nullptr
+ };
+ *reinterpret_cast<F**>(&buffer_) = new F(std::move(code));
+ vtable_ = &vtable;
+ }
+
+public:
+ template <class F> explicit Task(F code) { this->init(std::move(code)); }
+
+ operator bool() const { return vtable_ != nullptr; }
+ bool operator!() const { return vtable_ == nullptr; }
+
+ R operator()(Args... args)
+ {
+ if (vtable_ == nullptr)
+ throw std::bad_function_call();
+ const TaskVtable* vtable = vtable_;
+ vtable_ = nullptr;
+ // NOTE: std::forward<Args>(args)... is correct.
+ // see C++ [func.wrap.func.inv] for an example
+ return vtable->call(buffer_, std::forward<Args>(args)...);
+ }
+};
+
+template<class F, class... Args>
+class TaskImpl {
+ F code_;
+ std::tuple<Args...> args_;
+ using result_type = decltype(simgrid::xbt::apply(std::move(code_), std::move(args_)));
+
+public:
+ TaskImpl(F code, std::tuple<Args...> args) :
+ code_(std::move(code)),
+ args_(std::move(args))
+ {}
+ result_type operator()()
+ {
+ return simgrid::xbt::apply(std::move(code_), std::move(args_));
+ }
+};
+
+template <class F, class... Args> auto make_task(F code, Args... args) -> Task<decltype(code(std::move(args)...))()>
+{
+ TaskImpl<F, Args...> task(std::move(code), std::make_tuple(std::move(args)...));
+ return Task<decltype(code(std::move(args)...))()>(std::move(task));