#include <cstddef>
#include <cstdlib>
+#include <cstring>
#include <exception>
#include <functional>
#include <memory>
#include <string>
#include <tuple>
+#include <type_traits>
#include <utility>
#include <vector>
template<class T> class Task;
+namespace bits {
+
+ // Something similar exist in C++14:
+ template<class T>
+ constexpr T max(T a, T b)
+ {
+ return (a > b) ? a : b;
+ }
+ template<class T, class... Args>
+ constexpr T max(T a, Args... b)
+ {
+ return max(std::forward<T>(a), max(std::forward<Args>(b)...));
+ }
+
+ 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;
+ };
+ typedef char any_callback[max(
+ sizeof(ptr_callback),
+ sizeof(funcptr_callback),
+ sizeof(member_funcptr_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()
+ {
+ // 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);
+ }
+
+}
+
/** Type-erased run-once task
*
* * Like std::function but callable only once.
template<class R, class... Args>
class Task<R(Args...)> {
private:
- // Type-erasure for the code:
- class Base {
- public:
- virtual ~Base() {}
- virtual R operator()(Args...) = 0;
- };
- template<class F>
- class Impl : public Base {
- public:
- Impl(F&& code) : code_(std::move(code)) {}
- Impl(F const& code) : code_(code) {}
- ~Impl() override {}
- R operator()(Args... args) override
- {
- return code_(std::forward<Args>(args)...);
- }
- private:
- F code_;
+
+ typedef bits::TaskErasure TaskErasure;
+ struct TaskErasureVtable {
+ // Call (and possibly destroy) the function:
+ R (*call)(TaskErasure&, Args...);
+ // Destroy the function:
+ void (*destroy)(TaskErasure&);
};
- std::unique_ptr<Base> code_;
+
+ TaskErasure code_;
+ const TaskErasureVtable* vtable_ = nullptr;
+
public:
Task() {}
Task(std::nullptr_t) {}
+ ~Task()
+ {
+ if (vtable_ && vtable_->destroy)
+ vtable_->destroy(code_);
+ }
+
+ Task(Task const&) = delete;
+ Task& operator=(Task const&) = delete;
+
+ Task(Task&& that)
+ {
+ std::memcpy(&code_, &that.code_, sizeof(code_));
+ vtable_ = that.vtable_;
+ that.vtable_ = nullptr;
+ }
+ Task& operator=(Task&& that)
+ {
+ if (vtable_ && vtable_->destroy)
+ vtable_->destroy(code_);
+ std::memcpy(&code_, &that.code_, sizeof(code_));
+ vtable_ = that.vtable_;
+ that.vtable_ = nullptr;
+ return *this;
+ }
+
+ template<class F,
+ typename = typename std::enable_if<bits::isUsableDirectlyInTask<F>()>::type>
+ Task(F const& code)
+ {
+ const static TaskErasureVtable 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, sizeof(code.code));
+ code.code(std::forward<Args>(args)...);
+ },
+ // Destroy:
+ nullptr
+ };
+ if (!std::is_empty<F>::value)
+ std::memcpy(&code_.any, &code, sizeof(code));
+ vtable_ = &vtable;
+ }
+
+ template<class F,
+ typename = typename std::enable_if<!bits::isUsableDirectlyInTask<F>()>::type>
+ Task(F code)
+ {
+ const static TaskErasureVtable vtable {
+ // Call:
+ [](TaskErasure& erasure, Args... args) -> R {
+ // Delete F when we go out of scope:
+ std::unique_ptr<F> code(static_cast<F*>(erasure.ptr));
+ (*code)(std::forward<Args>(args)...);
+ },
+ // Destroy:
+ [](TaskErasure& erasure) {
+ F* code = static_cast<F*>(erasure.ptr);
+ delete code;
+ }
+ };
+ code_.ptr = new F(std::move(code));
+ vtable_ = &vtable;
+ }
template<class F>
- Task(F&& code) :
- code_(new Impl<F>(std::forward<F>(code))) {}
+ Task(std::reference_wrapper<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;
+ }
- operator bool() const { return code_ != nullptr; }
- bool operator!() const { return code_ == nullptr; }
+ operator bool() const { return vtable_ != nullptr; }
+ bool operator!() const { return vtable_ == nullptr; }
- template<class... OtherArgs>
- R operator()(OtherArgs&&... args)
+ R operator()(Args... args)
{
- std::unique_ptr<Base> code = std::move(code_);
- return (*code)(std::forward<OtherArgs>(args)...);
+ if (!vtable_)
+ throw std::bad_function_call();
+ const TaskErasureVtable* vtable = vtable_;
+ vtable_ = nullptr;
+ return vtable->call(code_, std::forward<Args>(args)...);
}
};
