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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>
85 #include <condition_variable>
87 #include <unordered_map>
92 #include <glog/logging.h>
96 // For actual usage, please see the Singleton<T> class at the bottom
97 // of this file; that is what you will actually interact with.
99 // SingletonVault is the class that manages singleton instances. It
100 // is unaware of the underlying types of singletons, and simply
101 // manages lifecycles and invokes CreateFunc and TeardownFunc when
102 // appropriate. In general, you won't need to interact with the
103 // SingletonVault itself.
105 // A vault goes through a few stages of life:
107 // 1. Registration phase; singletons can be registered, but no
108 // singleton can be created.
109 // 2. registrationComplete() has been called; singletons can no
110 // longer be registered, but they can be created.
111 // 3. A vault can return to stage 1 when destroyInstances is called.
113 // In general, you don't need to worry about any of the above; just
114 // ensure registrationComplete() is called near the top of your main()
115 // function, otherwise no singletons can be instantiated.
119 const char* const kDefaultTypeDescriptorName = "(default)";
120 // A TypeDescriptor is the unique handle for a given singleton. It is
121 // a combinaiton of the type and of the optional name, and is used as
122 // a key in unordered_maps.
123 class TypeDescriptor {
125 TypeDescriptor(const std::type_info& ti, std::string name)
126 : ti_(ti), name_(name) {
127 if (name_ == kDefaultTypeDescriptorName) {
128 LOG(DFATAL) << "Caller used the default name as their literal name; "
129 << "name your singleton something other than "
130 << kDefaultTypeDescriptorName;
134 std::string name() const {
135 std::string ret = ti_.name();
138 ret += kDefaultTypeDescriptorName;
145 friend class TypeDescriptorHasher;
147 bool operator==(const TypeDescriptor& other) const {
148 return ti_ == other.ti_ && name_ == other.name_;
152 const std::type_index ti_;
153 const std::string name_;
156 class TypeDescriptorHasher {
158 size_t operator()(const TypeDescriptor& ti) const {
159 return folly::hash::hash_combine(ti.ti_, ti.name_);
164 class SingletonVault {
166 enum class Type { Strict, Relaxed };
168 explicit SingletonVault(Type type = Type::Relaxed) : type_(type) {}
171 typedef std::function<void(void*)> TeardownFunc;
172 typedef std::function<void*(void)> CreateFunc;
174 // Register a singleton of a given type with the create and teardown
176 void registerSingleton(detail::TypeDescriptor type,
178 TeardownFunc teardown) {
179 std::lock_guard<std::mutex> guard(mutex_);
181 stateCheck(SingletonVaultState::Registering);
182 CHECK_THROW(singletons_.find(type) == singletons_.end(), std::logic_error);
183 auto& entry = singletons_[type];
185 entry.reset(new SingletonEntry);
188 std::lock_guard<std::mutex> entry_guard(entry->mutex);
189 CHECK(entry->instance == nullptr);
192 entry->create = create;
193 entry->teardown = teardown;
194 entry->state = SingletonEntryState::Dead;
197 // Mark registration is complete; no more singletons can be
198 // registered at this point.
199 void registrationComplete() {
200 std::lock_guard<std::mutex> guard(mutex_);
201 stateCheck(SingletonVaultState::Registering);
202 state_ = SingletonVaultState::Running;
205 // Destroy all singletons; when complete, the vault can create
206 // singletons once again, or remain dormant.
207 void destroyInstances();
209 // Retrieve a singleton from the vault, creating it if necessary.
210 std::shared_ptr<void> get_shared(detail::TypeDescriptor type) {
211 std::unique_lock<std::mutex> lock(mutex_);
212 auto entry = get_entry(type, &lock);
213 return entry->instance;
216 // This function is inherently racy since we don't hold the
217 // shared_ptr that contains the Singleton. It is the caller's
218 // responsibility to be sane with this, but it is preferable to use
219 // the weak_ptr interface for true safety.
220 void* get_ptr(detail::TypeDescriptor type) {
221 std::unique_lock<std::mutex> lock(mutex_);
222 auto entry = get_entry(type, &lock);
223 return entry->instance_ptr;
226 // For testing; how many registered and living singletons we have.
227 size_t registeredSingletonCount() const {
228 std::lock_guard<std::mutex> guard(mutex_);
229 return singletons_.size();
232 size_t livingSingletonCount() const {
233 std::lock_guard<std::mutex> guard(mutex_);
235 for (const auto& p : singletons_) {
236 if (p.second->instance) {
244 // A well-known vault; you can actually have others, but this is the
246 static SingletonVault* singleton();
249 // The two stages of life for a vault, as mentioned in the class comment.
250 enum class SingletonVaultState {
255 // Each singleton in the vault can be in three states: dead
256 // (registered but never created), being born (running the
257 // CreateFunc), and living (CreateFunc returned an instance).
258 enum class SingletonEntryState {
264 void stateCheck(SingletonVaultState expected,
265 const char* msg="Unexpected singleton state change") {
266 if (type_ == Type::Strict && expected != state_) {
267 throw std::logic_error(msg);
271 // An actual instance of a singleton, tracking the instance itself,
272 // its state as described above, and the create and teardown
274 struct SingletonEntry {
275 // mutex protects the entire entry
278 // state changes notify state_condvar
279 SingletonEntryState state = SingletonEntryState::Dead;
280 std::condition_variable state_condvar;
282 // the thread creating the singleton
283 std::thread::id creating_thread;
285 // The singleton itself and related functions.
286 std::shared_ptr<void> instance;
287 void* instance_ptr = nullptr;
288 CreateFunc create = nullptr;
289 TeardownFunc teardown = nullptr;
291 SingletonEntry() = default;
292 SingletonEntry(const SingletonEntry&) = delete;
293 SingletonEntry& operator=(const SingletonEntry&) = delete;
294 SingletonEntry& operator=(SingletonEntry&&) = delete;
295 SingletonEntry(SingletonEntry&&) = delete;
298 // Get a pointer to the living SingletonEntry for the specified
299 // type. The singleton is created as part of this function, if
301 SingletonEntry* get_entry(detail::TypeDescriptor type,
302 std::unique_lock<std::mutex>* lock) {
303 // mutex must be held when calling this function
305 SingletonVaultState::Running,
306 "Attempt to load a singleton before "
307 "SingletonVault::registrationComplete was called (hint: you probably "
308 "didn't call initFacebook)");
310 auto it = singletons_.find(type);
311 if (it == singletons_.end()) {
312 throw std::out_of_range(std::string("non-existent singleton: ") +
316 auto entry = it->second.get();
317 std::unique_lock<std::mutex> entry_lock(entry->mutex);
319 if (entry->state == SingletonEntryState::BeingBorn) {
320 // If this thread is trying to give birth to the singleton, it's
321 // a circular dependency and we must panic.
322 if (entry->creating_thread == std::this_thread::get_id()) {
323 throw std::out_of_range(std::string("circular singleton dependency: ") +
327 // Otherwise, another thread is constructing the singleton;
328 // let's wait on a condvar to see it complete. We release and
329 // reaquire lock while waiting on the entry to resolve its state.
331 entry->state_condvar.wait(entry_lock, [&entry]() {
332 return entry->state != SingletonEntryState::BeingBorn;
337 if (entry->instance == nullptr) {
338 CHECK(entry->state == SingletonEntryState::Dead);
339 entry->state = SingletonEntryState::BeingBorn;
340 entry->creating_thread = std::this_thread::get_id();
344 // Can't use make_shared -- no support for a custom deleter, sadly.
345 auto instance = std::shared_ptr<void>(entry->create(), entry->teardown);
349 CHECK(entry->state == SingletonEntryState::BeingBorn);
350 entry->instance = instance;
351 entry->instance_ptr = instance.get();
352 entry->state = SingletonEntryState::Living;
353 entry->state_condvar.notify_all();
355 creation_order_.push_back(type);
357 CHECK(entry->state == SingletonEntryState::Living);
361 mutable std::mutex mutex_;
362 typedef std::unique_ptr<SingletonEntry> SingletonEntryPtr;
363 std::unordered_map<detail::TypeDescriptor,
365 detail::TypeDescriptorHasher> singletons_;
366 std::vector<detail::TypeDescriptor> creation_order_;
367 SingletonVaultState state_ = SingletonVaultState::Registering;
368 Type type_ = Type::Relaxed;
371 // This is the wrapper class that most users actually interact with.
372 // It allows for simple access to registering and instantiating
373 // singletons. Create instances of this class in the global scope of
374 // type Singleton<T> to register your singleton for later access via
375 // Singleton<T>::get().
376 template <typename T>
379 typedef std::function<T*(void)> CreateFunc;
380 typedef std::function<void(T*)> TeardownFunc;
382 // Generally your program life cycle should be fine with calling
383 // get() repeatedly rather than saving the reference, and then not
384 // call get() during process shutdown.
385 static T* get(SingletonVault* vault = nullptr /* for testing */) {
386 return get_ptr({typeid(T), ""}, vault);
389 static T* get(const char* name,
390 SingletonVault* vault = nullptr /* for testing */) {
391 return get_ptr({typeid(T), name}, vault);
394 // If, however, you do need to hold a reference to the specific
395 // singleton, you can try to do so with a weak_ptr. Avoid this when
396 // possible but the inability to lock the weak pointer can be a
397 // signal that the vault has been destroyed.
398 static std::weak_ptr<T> get_weak(
399 SingletonVault* vault = nullptr /* for testing */) {
400 return get_weak("", vault);
403 static std::weak_ptr<T> get_weak(
404 const char* name, SingletonVault* vault = nullptr /* for testing */) {
405 return std::weak_ptr<T>(get_shared({typeid(T), name}, vault));
408 std::weak_ptr<T> get_weak(const char* name) {
409 return std::weak_ptr<T>(get_shared({typeid(T), name}, vault_));
412 // Allow the Singleton<t> instance to also retrieve the underlying
413 // singleton, if desired.
414 T* ptr() { return get_ptr(type_descriptor_, vault_); }
415 T& operator*() { return *ptr(); }
416 T* operator->() { return ptr(); }
418 explicit Singleton(Singleton::CreateFunc c = nullptr,
419 Singleton::TeardownFunc t = nullptr,
420 SingletonVault* vault = nullptr /* for testing */)
421 : Singleton({typeid(T), ""}, c, t, vault) {}
423 explicit Singleton(const char* name,
424 Singleton::CreateFunc c = nullptr,
425 Singleton::TeardownFunc t = nullptr,
426 SingletonVault* vault = nullptr /* for testing */)
427 : Singleton({typeid(T), name}, c, t, vault) {}
430 explicit Singleton(detail::TypeDescriptor type,
431 Singleton::CreateFunc c = nullptr,
432 Singleton::TeardownFunc t = nullptr,
433 SingletonVault* vault = nullptr /* for testing */)
434 : type_descriptor_(type) {
436 c = []() { return new T; };
438 SingletonVault::TeardownFunc teardown;
440 teardown = [](void* v) { delete static_cast<T*>(v); };
442 teardown = [t](void* v) { t(static_cast<T*>(v)); };
445 if (vault == nullptr) {
446 vault = SingletonVault::singleton();
449 vault->registerSingleton(type, c, teardown);
452 static T* get_ptr(detail::TypeDescriptor type_descriptor = {typeid(T), ""},
453 SingletonVault* vault = nullptr /* for testing */) {
454 return static_cast<T*>(
455 (vault ?: SingletonVault::singleton())->get_ptr(type_descriptor));
458 // Don't use this function, it's private for a reason! Using it
459 // would defeat the *entire purpose* of the vault in that we lose
460 // the ability to guarantee that, after a destroyInstances is
461 // called, all instances are, in fact, destroyed. You should use
462 // weak_ptr if you need to hold a reference to the singleton and
463 // guarantee briefly that it exists.
465 // Yes, you can just get the weak pointer and lock it, but hopefully
466 // if you have taken the time to read this far, you see why that
468 static std::shared_ptr<T> get_shared(
469 detail::TypeDescriptor type_descriptor = {typeid(T), ""},
470 SingletonVault* vault = nullptr /* for testing */) {
471 return std::static_pointer_cast<T>(
472 (vault ?: SingletonVault::singleton())->get_shared(type_descriptor));
475 detail::TypeDescriptor type_descriptor_;
476 SingletonVault* vault_;