template<class T> class Task;
-namespace bits {
-
- // Compute the maximum size taken by any of the given types:
- template <class... T> struct max_size;
- template <>
- struct max_size<> : public std::integral_constant<std::size_t, 0> {};
- template <class T>
- struct max_size<T> : public std::integral_constant<std::size_t, sizeof(T)> {};
- template <class T, class... U>
- struct max_size<T, U...> : public std::integral_constant<std::size_t,
- (sizeof(T) > max_size<U...>::value) ? sizeof(T) : max_size<U...>::value
- > {};
-
- struct whatever {};
-
- // What we can store in a Task:
- typedef void* ptr_callback;
- struct funcptr_callback {
- // Placeholder for any function pointer:
- void(*callback)();
- void* data;
- };
- struct member_funcptr_callback {
- // Placeholder for any pointer to member function:
- void (whatever::* callback)();
- whatever* data;
- };
- constexpr std::size_t any_size = max_size<
- ptr_callback,
- funcptr_callback,
- member_funcptr_callback
- >::value;
- typedef std::array<char, any_size> any_callback;
-
- // Union of what we can store in a Task:
- union TaskErasure {
- ptr_callback ptr;
- funcptr_callback funcptr;
- member_funcptr_callback member_funcptr;
- any_callback any;
- };
-
- // Can we copy F in Task (or do we have to use the heap)?
- template<class F>
- constexpr bool isUsableDirectlyInTask()
- {
- // TODO, detect availability std::is_trivially_copyable / workaround
-#if 1
- // std::is_trivially_copyable is not available before GCC 5.
- return false;
-#else
- // The only types we can portably store directly in the Task are the
- // trivially copyable ones (we can memcpy) which are small enough to fit:
- return std::is_trivially_copyable<F>::value &&
- sizeof(F) <= sizeof(bits::any_callback);
-#endif
- }
-
-}
-
/** Type-erased run-once task
*
* * Like std::function but callable only once.
class Task<R(Args...)> {
private:
- typedef bits::TaskErasure TaskErasure;
- struct TaskErasureVtable {
- // Call (and possibly destroy) the function:
- R (*call)(TaskErasure&, Args...);
- // Destroy the function:
- void (*destroy)(TaskErasure&);
+ // Placeholder for some class type:
+ struct whatever {};
+
+ // Union used for storage:
+#if 0
+ typedef typename std::aligned_union<0,
+ void*,
+ std::pair<void(*)(),void*>,
+ std::pair<void(whatever::*)(), whatever*>
+ >::type TaskUnion;
+#else
+ union TaskUnion {
+ void* ptr;
+ std::pair<void(*)(),void*> funcptr;
+ std::pair<void(whatever::*)(), whatever*> memberptr;
+ char any1[sizeof(std::pair<void(*)(),void*>)];
+ char any2[sizeof(std::pair<void(whatever::*)(), whatever*>)];
+ TaskUnion() {}
+ ~TaskUnion() {}
+ };
+#endif
+
+ // 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:
+ typedef R (*call_function)(TaskUnion&, Args...);
+ // Destroy the function (of needed):
+ typedef void (*destroy_function)(TaskUnion&);
+ // Move the function (otherwise memcpy):
+ typedef void (*move_function)(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;
};
- TaskErasure code_;
- const TaskErasureVtable* vtable_ = nullptr;
+ TaskUnion buffer_;
+ const TaskVtable* vtable_ = nullptr;
+
+ void clear()
+ {
+ if (vtable_ && vtable_->destroy)
+ vtable_->destroy(buffer_);
+ }
public:
+
Task() {}
Task(std::nullptr_t) {}
~Task()
{
- if (vtable_ && vtable_->destroy)
- vtable_->destroy(code_);
+ this->clear();
}
Task(Task const&) = delete;
- Task& operator=(Task const&) = delete;
Task(Task&& that)
{
- std::memcpy(&code_, &that.code_, sizeof(code_));
+ if (that.vtable_ && that.vtable_->move)
+ that.vtable_->move(buffer_, that.buffer_);
+ else
+ std::memcpy(&buffer_, &that.buffer_, sizeof(buffer_));
vtable_ = that.vtable_;
that.vtable_ = nullptr;
}
- Task& operator=(Task&& that)
+ Task& operator=(Task that)
{
- if (vtable_ && vtable_->destroy)
- vtable_->destroy(code_);
- std::memcpy(&code_, &that.code_, sizeof(code_));
+ this->clear();
+ if (that.vtable_ && that.vtable_->move)
+ that.vtable_->move(buffer_, that.buffer_);
+ else
+ std::memcpy(&buffer_, &that.buffer_, sizeof(buffer_));
vtable_ = that.vtable_;
that.vtable_ = nullptr;
return *this;
}
- template<class F,
- typename = typename std::enable_if<bits::isUsableDirectlyInTask<F>()>::type>
- Task(F const& code)
+private:
+
+ template<class F>
+ typename std::enable_if<canSBO<F>()>::type
+ init(F code)
{
- const static TaskErasureVtable vtable {
+ const static TaskVtable vtable {
// Call:
- [](TaskErasure& erasure, Args... args) -> R {
- // We need to wrap F un a union because F might not have a default
- // constructor: this is especially the case for lambdas.
- union no_ctor {
- no_ctor() {}
- ~no_ctor() {}
- F code ;
- } code;
- if (!std::is_empty<F>::value)
- // AFAIU, this is safe as per [basic.types]:
- std::memcpy(&code.code, erasure.any.data(), sizeof(code.code));
- code.code(std::forward<Args>(args)...);
+ [](TaskUnion& buffer, Args... args) -> R {
+ F* src = reinterpret_cast<F*>(&buffer);
+ F code = std::move(*src);
+ src->~F();
+ code(std::forward<Args>(args)...);
},
// Destroy:
- nullptr
+ std::is_trivially_destructible<F>::value ?
+ static_cast<destroy_function>(nullptr) :
+ [](TaskUnion& buffer) {
+ F* code = reinterpret_cast<F*>(&buffer);
+ code->~F();
+ },
+ // Move:
+ [](TaskUnion& dst, TaskUnion& src) {
+ F* src_code = reinterpret_cast<F*>(&src);
+ F* dst_code = reinterpret_cast<F*>(&dst);
+ new(dst_code) F(std::move(*src_code));
+ src_code->~F();
+ }
};
- if (!std::is_empty<F>::value)
- std::memcpy(code_.any.data(), &code, sizeof(code));
+ new(&buffer_) F(std::move(code));
vtable_ = &vtable;
}
- template<class F,
- typename = typename std::enable_if<!bits::isUsableDirectlyInTask<F>()>::type>
- Task(F code)
+ template<class F>
+ typename std::enable_if<!canSBO<F>()>::type
+ init(F code)
{
- const static TaskErasureVtable vtable {
+ const static TaskVtable vtable {
// Call:
- [](TaskErasure& erasure, Args... args) -> R {
+ [](TaskUnion& buffer, Args... args) -> R {
// Delete F when we go out of scope:
- std::unique_ptr<F> code(static_cast<F*>(erasure.ptr));
+ std::unique_ptr<F> code(*reinterpret_cast<F**>(&buffer));
(*code)(std::forward<Args>(args)...);
},
// Destroy:
- [](TaskErasure& erasure) {
- F* code = static_cast<F*>(erasure.ptr);
+ [](TaskUnion& buffer) {
+ F* code = *reinterpret_cast<F**>(&buffer);
delete code;
- }
+ },
+ // Move:
+ nullptr
};
- code_.ptr = new F(std::move(code));
+ *reinterpret_cast<F**>(&buffer_) = new F(std::move(code));
vtable_ = &vtable;
}
+public:
+
template<class F>
- Task(std::reference_wrapper<F> code)
+ Task(F code)
{
- const static TaskErasureVtable vtable {
- // Call:
- [](TaskErasure& erasure, Args... args) -> R {
- F* code = static_cast<F*>(erasure.ptr);
- (*code)(std::forward<Args>(args)...);
- },
- // Destroy:
- nullptr
- };
- code.code_.ptr = code.get();
- vtable_ = &vtable;
+ this->init(std::move(code));
}
- // TODO, Task(funcptr)
- // TODO, Task(funcptr, data)
- // TODO, Task(method, object)
- // TODO, Task(stateless lambda)
-
operator bool() const { return vtable_ != nullptr; }
bool operator!() const { return vtable_ == nullptr; }
R operator()(Args... args)
{
- if (!vtable_)
+ if (vtable_ == nullptr)
throw std::bad_function_call();
- const TaskErasureVtable* vtable = vtable_;
+ const TaskVtable* vtable = vtable_;
vtable_ = nullptr;
- return vtable->call(
code_, std::forward<Args>(args)...);
+ return vtable->call(
buffer_, std::forward<Args>(args)...);
}
};
