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17 // SingletonVault - a library to manage the creation and destruction
18 // of interdependent singletons.
20 // Basic usage of this class is very simple; suppose you have a class
21 // called MyExpensiveService, and you only want to construct one (ie,
22 // it's a singleton), but you only want to construct it if it is used.
25 // class MyExpensiveService { ... };
28 // namespace { folly::Singleton<MyExpensiveService> the_singleton; }
30 // Code can access it via:
32 // MyExpensiveService* instance = Singleton<MyExpensiveService>::get();
34 // std::weak_ptr<MyExpensiveService> instance =
35 // Singleton<MyExpensiveService>::get_weak();
37 // You also can directly access it by the variable defining the
38 // singleton rather than via get(), and even treat that variable like
39 // a smart pointer (dereferencing it or using the -> operator).
41 // Please note, however, that all non-weak_ptr interfaces are
42 // inherently subject to races with destruction. Use responsibly.
44 // The singleton will be created on demand. If the constructor for
45 // MyExpensiveService actually makes use of *another* Singleton, then
46 // the right thing will happen -- that other singleton will complete
47 // construction before get() returns. However, in the event of a
48 // circular dependency, a runtime error will occur.
50 // You can have multiple singletons of the same underlying type, but
51 // each must be given a unique name:
54 // folly::Singleton<MyExpensiveService> s1("name1");
55 // folly::Singleton<MyExpensiveService> s2("name2");
58 // MyExpensiveService* svc1 = Singleton<MyExpensiveService>::get("name1");
59 // MyExpensiveService* svc2 = Singleton<MyExpensiveService>::get("name2");
61 // By default, the singleton instance is constructed via new and
62 // deleted via delete, but this is configurable:
64 // namespace { folly::Singleton<MyExpensiveService> the_singleton(create,
67 // Where create and destroy are functions, Singleton<T>::CreateFunc
68 // Singleton<T>::TeardownFunc.
70 // What if you need to destroy all of your singletons? Say, some of
71 // your singletons manage threads, but you need to fork? Or your unit
72 // test wants to clean up all global state? Then you can call
73 // SingletonVault::singleton()->destroyInstances(), which invokes the
74 // TeardownFunc for each singleton, in the reverse order they were
75 // created. It is your responsibility to ensure your singletons can
76 // handle cases where the singletons they depend on go away, however.
79 #include <folly/Exception.h>
80 #include <folly/Hash.h>
81 #include <folly/RWSpinLock.h>
86 #include <condition_variable>
88 #include <unordered_map>
93 #include <glog/logging.h>
97 // For actual usage, please see the Singleton<T> class at the bottom
98 // of this file; that is what you will actually interact with.
100 // SingletonVault is the class that manages singleton instances. It
101 // is unaware of the underlying types of singletons, and simply
102 // manages lifecycles and invokes CreateFunc and TeardownFunc when
103 // appropriate. In general, you won't need to interact with the
104 // SingletonVault itself.
106 // A vault goes through a few stages of life:
108 // 1. Registration phase; singletons can be registered, but no
109 // singleton can be created.
110 // 2. registrationComplete() has been called; singletons can no
111 // longer be registered, but they can be created.
112 // 3. A vault can return to stage 1 when destroyInstances is called.
114 // In general, you don't need to worry about any of the above; just
115 // ensure registrationComplete() is called near the top of your main()
116 // function, otherwise no singletons can be instantiated.
120 const char* const kDefaultTypeDescriptorName = "(default)";
121 // A TypeDescriptor is the unique handle for a given singleton. It is
122 // a combinaiton of the type and of the optional name, and is used as
123 // a key in unordered_maps.
124 class TypeDescriptor {
126 TypeDescriptor(const std::type_info& ti, std::string name)
127 : ti_(ti), name_(name) {
128 if (name_ == kDefaultTypeDescriptorName) {
129 LOG(DFATAL) << "Caller used the default name as their literal name; "
130 << "name your singleton something other than "
131 << kDefaultTypeDescriptorName;
135 std::string name() const {
136 std::string ret = ti_.name();
139 ret += kDefaultTypeDescriptorName;
146 friend class TypeDescriptorHasher;
148 bool operator==(const TypeDescriptor& other) const {
149 return ti_ == other.ti_ && name_ == other.name_;
153 const std::type_index ti_;
154 const std::string name_;
157 class TypeDescriptorHasher {
159 size_t operator()(const TypeDescriptor& ti) const {
160 return folly::hash::hash_combine(ti.ti_, ti.name_);
165 class SingletonVault {
167 enum class Type { Strict, Relaxed };
169 explicit SingletonVault(Type type = Type::Relaxed) : type_(type) {}
171 // Destructor is only called by unit tests to check destroyInstances.
174 typedef std::function<void(void*)> TeardownFunc;
175 typedef std::function<void*(void)> CreateFunc;
177 // Register a singleton of a given type with the create and teardown
179 void registerSingleton(detail::TypeDescriptor type,
181 TeardownFunc teardown) {
182 RWSpinLock::WriteHolder wh(&mutex_);
184 stateCheck(SingletonVaultState::Registering);
185 CHECK_THROW(singletons_.find(type) == singletons_.end(), std::logic_error);
186 auto& entry = singletons_[type];
187 entry.reset(new SingletonEntry);
189 std::lock_guard<std::mutex> entry_guard(entry->mutex);
190 CHECK(entry->instance == nullptr);
193 entry->create = create;
194 entry->teardown = teardown;
195 entry->state = SingletonEntryState::Dead;
198 // Mark registration is complete; no more singletons can be
199 // registered at this point.
200 void registrationComplete() {
201 RWSpinLock::WriteHolder wh(&mutex_);
203 stateCheck(SingletonVaultState::Registering);
204 state_ = SingletonVaultState::Running;
207 // Destroy all singletons; when complete, the vault can create
208 // singletons once again, or remain dormant.
209 void destroyInstances();
211 // Retrieve a singleton from the vault, creating it if necessary.
212 std::shared_ptr<void> get_shared(detail::TypeDescriptor type) {
213 auto entry = get_entry_create(type);
214 return entry->instance;
217 // This function is inherently racy since we don't hold the
218 // shared_ptr that contains the Singleton. It is the caller's
219 // responsibility to be sane with this, but it is preferable to use
220 // the weak_ptr interface for true safety.
221 void* get_ptr(detail::TypeDescriptor type) {
222 auto entry = get_entry_create(type);
223 return entry->instance_ptr;
226 // For testing; how many registered and living singletons we have.
227 size_t registeredSingletonCount() const {
228 RWSpinLock::ReadHolder rh(&mutex_);
230 return singletons_.size();
233 size_t livingSingletonCount() const {
234 RWSpinLock::ReadHolder rh(&mutex_);
237 for (const auto& p : singletons_) {
238 std::lock_guard<std::mutex> entry_guard(p.second->mutex);
239 if (p.second->instance) {
247 // A well-known vault; you can actually have others, but this is the
249 static SingletonVault* singleton();
252 // The two stages of life for a vault, as mentioned in the class comment.
253 enum class SingletonVaultState {
258 // Each singleton in the vault can be in three states: dead
259 // (registered but never created), being born (running the
260 // CreateFunc), and living (CreateFunc returned an instance).
261 enum class SingletonEntryState {
267 void stateCheck(SingletonVaultState expected,
268 const char* msg="Unexpected singleton state change") {
269 if (type_ == Type::Strict && expected != state_) {
270 throw std::logic_error(msg);
274 // An actual instance of a singleton, tracking the instance itself,
275 // its state as described above, and the create and teardown
277 struct SingletonEntry {
278 // mutex protects the entire entry
281 // state changes notify state_condvar
282 SingletonEntryState state = SingletonEntryState::Dead;
283 std::condition_variable state_condvar;
285 // the thread creating the singleton
286 std::thread::id creating_thread;
288 // The singleton itself and related functions.
289 std::shared_ptr<void> instance;
290 void* instance_ptr = nullptr;
291 CreateFunc create = nullptr;
292 TeardownFunc teardown = nullptr;
294 SingletonEntry() = default;
295 SingletonEntry(const SingletonEntry&) = delete;
296 SingletonEntry& operator=(const SingletonEntry&) = delete;
297 SingletonEntry& operator=(SingletonEntry&&) = delete;
298 SingletonEntry(SingletonEntry&&) = delete;
301 SingletonEntry* get_entry(detail::TypeDescriptor type) {
302 RWSpinLock::ReadHolder rh(&mutex_);
304 // mutex must be held when calling this function
306 SingletonVaultState::Running,
307 "Attempt to load a singleton before "
308 "SingletonVault::registrationComplete was called (hint: you probably "
309 "didn't call initFacebook)");
311 auto it = singletons_.find(type);
312 if (it == singletons_.end()) {
313 throw std::out_of_range(std::string("non-existent singleton: ") +
317 return it->second.get();
320 // Get a pointer to the living SingletonEntry for the specified
321 // type. The singleton is created as part of this function, if
323 SingletonEntry* get_entry_create(detail::TypeDescriptor type) {
324 auto entry = get_entry(type);
326 std::unique_lock<std::mutex> entry_lock(entry->mutex);
328 if (entry->state == SingletonEntryState::BeingBorn) {
329 // If this thread is trying to give birth to the singleton, it's
330 // a circular dependency and we must panic.
331 if (entry->creating_thread == std::this_thread::get_id()) {
332 throw std::out_of_range(std::string("circular singleton dependency: ") +
336 entry->state_condvar.wait(entry_lock, [&entry]() {
337 return entry->state != SingletonEntryState::BeingBorn;
341 if (entry->instance == nullptr) {
342 CHECK(entry->state == SingletonEntryState::Dead);
343 entry->state = SingletonEntryState::BeingBorn;
344 entry->creating_thread = std::this_thread::get_id();
347 // Can't use make_shared -- no support for a custom deleter, sadly.
348 auto instance = std::shared_ptr<void>(entry->create(), entry->teardown);
351 CHECK(entry->state == SingletonEntryState::BeingBorn);
352 entry->instance = instance;
353 entry->instance_ptr = instance.get();
354 entry->state = SingletonEntryState::Living;
355 entry->state_condvar.notify_all();
358 RWSpinLock::WriteHolder wh(&mutex_);
360 creation_order_.push_back(type);
363 CHECK(entry->state == SingletonEntryState::Living);
367 mutable folly::RWSpinLock mutex_;
368 typedef std::unique_ptr<SingletonEntry> SingletonEntryPtr;
369 std::unordered_map<detail::TypeDescriptor,
371 detail::TypeDescriptorHasher> singletons_;
372 std::vector<detail::TypeDescriptor> creation_order_;
373 SingletonVaultState state_ = SingletonVaultState::Registering;
374 Type type_ = Type::Relaxed;
377 // This is the wrapper class that most users actually interact with.
378 // It allows for simple access to registering and instantiating
379 // singletons. Create instances of this class in the global scope of
380 // type Singleton<T> to register your singleton for later access via
381 // Singleton<T>::get().
382 template <typename T>
385 typedef std::function<T*(void)> CreateFunc;
386 typedef std::function<void(T*)> TeardownFunc;
388 // Generally your program life cycle should be fine with calling
389 // get() repeatedly rather than saving the reference, and then not
390 // call get() during process shutdown.
391 static T* get(SingletonVault* vault = nullptr /* for testing */) {
392 return get_ptr({typeid(T), ""}, vault);
395 static T* get(const char* name,
396 SingletonVault* vault = nullptr /* for testing */) {
397 return get_ptr({typeid(T), name}, vault);
400 // If, however, you do need to hold a reference to the specific
401 // singleton, you can try to do so with a weak_ptr. Avoid this when
402 // possible but the inability to lock the weak pointer can be a
403 // signal that the vault has been destroyed.
404 static std::weak_ptr<T> get_weak(
405 SingletonVault* vault = nullptr /* for testing */) {
406 return get_weak("", vault);
409 static std::weak_ptr<T> get_weak(
410 const char* name, SingletonVault* vault = nullptr /* for testing */) {
411 return std::weak_ptr<T>(get_shared({typeid(T), name}, vault));
414 // Allow the Singleton<t> instance to also retrieve the underlying
415 // singleton, if desired.
416 T* ptr() { return get_ptr(type_descriptor_, vault_); }
417 T& operator*() { return *ptr(); }
418 T* operator->() { return ptr(); }
420 template <typename CreateFunc = std::nullptr_t>
421 explicit Singleton(CreateFunc c = nullptr,
422 Singleton::TeardownFunc t = nullptr,
423 SingletonVault* vault = nullptr /* for testing */)
424 : Singleton({typeid(T), ""}, c, t, vault) {}
426 template <typename CreateFunc = std::nullptr_t>
427 explicit Singleton(const char* name,
428 CreateFunc c = nullptr,
429 Singleton::TeardownFunc t = nullptr,
430 SingletonVault* vault = nullptr /* for testing */)
431 : Singleton({typeid(T), name}, c, t, vault) {}
434 explicit Singleton(detail::TypeDescriptor type,
436 Singleton::TeardownFunc t,
437 SingletonVault* vault) :
439 []() { return new T; },
444 explicit Singleton(detail::TypeDescriptor type,
445 Singleton::CreateFunc c,
446 Singleton::TeardownFunc t,
447 SingletonVault* vault)
448 : type_descriptor_(type) {
450 throw std::logic_error(
451 "nullptr_t should be passed if you want T to be default constructed");
453 SingletonVault::TeardownFunc teardown;
455 teardown = [](void* v) { delete static_cast<T*>(v); };
457 teardown = [t](void* v) { t(static_cast<T*>(v)); };
460 if (vault == nullptr) {
461 vault = SingletonVault::singleton();
464 vault->registerSingleton(type, c, teardown);
467 static T* get_ptr(detail::TypeDescriptor type_descriptor = {typeid(T), ""},
468 SingletonVault* vault = nullptr /* for testing */) {
469 return static_cast<T*>(
470 (vault ?: SingletonVault::singleton())->get_ptr(type_descriptor));
473 // Don't use this function, it's private for a reason! Using it
474 // would defeat the *entire purpose* of the vault in that we lose
475 // the ability to guarantee that, after a destroyInstances is
476 // called, all instances are, in fact, destroyed. You should use
477 // weak_ptr if you need to hold a reference to the singleton and
478 // guarantee briefly that it exists.
480 // Yes, you can just get the weak pointer and lock it, but hopefully
481 // if you have taken the time to read this far, you see why that
483 static std::shared_ptr<T> get_shared(
484 detail::TypeDescriptor type_descriptor = {typeid(T), ""},
485 SingletonVault* vault = nullptr /* for testing */) {
486 return std::static_pointer_cast<T>(
487 (vault ?: SingletonVault::singleton())->get_shared(type_descriptor));
490 detail::TypeDescriptor type_descriptor_;
491 SingletonVault* vault_;