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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 <unordered_map>
90 #include <glog/logging.h>
94 // For actual usage, please see the Singleton<T> class at the bottom
95 // of this file; that is what you will actually interact with.
97 // SingletonVault is the class that manages singleton instances. It
98 // is unaware of the underlying types of singletons, and simply
99 // manages lifecycles and invokes CreateFunc and TeardownFunc when
100 // appropriate. In general, you won't need to interact with the
101 // SingletonVault itself.
103 // A vault goes through a few stages of life:
105 // 1. Registration phase; singletons can be registered, but no
106 // singleton can be created.
107 // 2. registrationComplete() has been called; singletons can no
108 // longer be registered, but they can be created.
109 // 3. A vault can return to stage 1 when destroyInstances is called.
111 // In general, you don't need to worry about any of the above; just
112 // ensure registrationComplete() is called near the top of your main()
113 // function, otherwise no singletons can be instantiated.
117 const char* const kDefaultTypeDescriptorName = "(default)";
118 // A TypeDescriptor is the unique handle for a given singleton. It is
119 // a combinaiton of the type and of the optional name, and is used as
120 // a key in unordered_maps.
121 class TypeDescriptor {
123 TypeDescriptor(const std::type_info& ti, std::string name)
124 : ti_(ti), name_(name) {
125 if (name_ == kDefaultTypeDescriptorName) {
126 LOG(DFATAL) << "Caller used the default name as their literal name; "
127 << "name your singleton something other than "
128 << kDefaultTypeDescriptorName;
132 std::string name() const {
133 std::string ret = ti_.name();
136 ret += kDefaultTypeDescriptorName;
143 friend class TypeDescriptorHasher;
145 bool operator==(const TypeDescriptor& other) const {
146 return ti_ == other.ti_ && name_ == other.name_;
150 const std::type_index ti_;
151 const std::string name_;
154 class TypeDescriptorHasher {
156 size_t operator()(const TypeDescriptor& ti) const {
157 return folly::hash::hash_combine(ti.ti_, ti.name_);
162 class SingletonVault {
167 typedef std::function<void(void*)> TeardownFunc;
168 typedef std::function<void*(void)> CreateFunc;
170 // Register a singleton of a given type with the create and teardown
172 void registerSingleton(detail::TypeDescriptor type,
174 TeardownFunc teardown) {
175 std::lock_guard<std::mutex> guard(mutex_);
177 CHECK_THROW(state_ == SingletonVaultState::Registering, std::logic_error);
178 CHECK_THROW(singletons_.find(type) == singletons_.end(), std::logic_error);
179 auto& entry = singletons_[type];
181 entry.reset(new SingletonEntry);
184 std::lock_guard<std::mutex> entry_guard(entry->mutex_);
185 CHECK(entry->instance == nullptr);
188 entry->create = create;
189 entry->teardown = teardown;
190 entry->state = SingletonEntryState::Dead;
193 // Mark registration is complete; no more singletons can be
194 // registered at this point.
195 void registrationComplete() {
196 std::lock_guard<std::mutex> guard(mutex_);
197 CHECK_THROW(state_ == SingletonVaultState::Registering, std::logic_error);
198 state_ = SingletonVaultState::Running;
201 // Destroy all singletons; when complete, the vault can create
202 // singletons once again, or remain dormant.
203 void destroyInstances();
205 // Retrieve a singleton from the vault, creating it if necessary.
206 std::shared_ptr<void> get_shared(detail::TypeDescriptor type) {
207 std::unique_lock<std::mutex> lock(mutex_);
208 auto entry = get_entry(type, &lock);
209 return entry->instance;
212 // This function is inherently racy since we don't hold the
213 // shared_ptr that contains the Singleton. It is the caller's
214 // responsibility to be sane with this, but it is preferable to use
215 // the weak_ptr interface for true safety.
216 void* get_ptr(detail::TypeDescriptor type) {
217 std::unique_lock<std::mutex> lock(mutex_);
218 auto entry = get_entry(type, &lock);
219 return entry->instance_ptr;
222 // For testing; how many registered and living singletons we have.
223 size_t registeredSingletonCount() const {
224 std::lock_guard<std::mutex> guard(mutex_);
225 return singletons_.size();
228 size_t livingSingletonCount() const {
229 std::lock_guard<std::mutex> guard(mutex_);
231 for (const auto& p : singletons_) {
232 if (p.second->instance) {
240 // A well-known vault; you can actually have others, but this is the
242 static SingletonVault* singleton();
245 // The two stages of life for a vault, as mentioned in the class comment.
246 enum class SingletonVaultState {
251 // Each singleton in the vault can be in three states: dead
252 // (registered but never created), being born (running the
253 // CreateFunc), and living (CreateFunc returned an instance).
254 enum class SingletonEntryState {
260 // An actual instance of a singleton, tracking the instance itself,
261 // its state as described above, and the create and teardown
263 struct SingletonEntry {
265 std::shared_ptr<void> instance;
266 void* instance_ptr = nullptr;
267 CreateFunc create = nullptr;
268 TeardownFunc teardown = nullptr;
269 SingletonEntryState state = SingletonEntryState::Dead;
271 SingletonEntry() = default;
272 SingletonEntry(const SingletonEntry&) = delete;
273 SingletonEntry& operator=(const SingletonEntry&) = delete;
274 SingletonEntry& operator=(SingletonEntry&&) = delete;
275 SingletonEntry(SingletonEntry&&) = delete;
278 // Get a pointer to the living SingletonEntry for the specified
279 // type. The singleton is created as part of this function, if
281 SingletonEntry* get_entry(detail::TypeDescriptor type,
282 std::unique_lock<std::mutex>* lock) {
283 // mutex_ must be held when calling this function
284 if (state_ != SingletonVaultState::Running) {
285 throw std::logic_error(
286 "Attempt to load a singleton before "
287 "SingletonVault::registrationComplete was called (hint: you probably "
288 "didn't call initFacebook)");
291 auto it = singletons_.find(type);
292 if (it == singletons_.end()) {
293 throw std::out_of_range(std::string("non-existent singleton: ") +
297 auto& entry = it->second;
298 std::unique_lock<std::mutex> entry_lock(entry->mutex_);
300 if (entry->state == SingletonEntryState::BeingBorn) {
301 throw std::out_of_range(std::string("circular singleton dependency: ") +
305 if (entry->instance == nullptr) {
306 CHECK(entry->state == SingletonEntryState::Dead);
307 entry->state = SingletonEntryState::BeingBorn;
311 // Can't use make_shared -- no support for a custom deleter, sadly.
312 auto instance = std::shared_ptr<void>(entry->create(), entry->teardown);
316 CHECK(entry->state == SingletonEntryState::BeingBorn);
317 entry->instance = instance;
318 entry->instance_ptr = instance.get();
319 entry->state = SingletonEntryState::Living;
321 creation_order_.push_back(type);
323 CHECK(entry->state == SingletonEntryState::Living);
327 mutable std::mutex mutex_;
328 typedef std::unique_ptr<SingletonEntry> SingletonEntryPtr;
329 std::unordered_map<detail::TypeDescriptor,
331 detail::TypeDescriptorHasher> singletons_;
332 std::vector<detail::TypeDescriptor> creation_order_;
333 SingletonVaultState state_ = SingletonVaultState::Registering;
336 // This is the wrapper class that most users actually interact with.
337 // It allows for simple access to registering and instantiating
338 // singletons. Create instances of this class in the global scope of
339 // type Singleton<T> to register your singleton for later access via
340 // Singleton<T>::get().
341 template <typename T>
344 typedef std::function<T*(void)> CreateFunc;
345 typedef std::function<void(T*)> TeardownFunc;
347 // Generally your program life cycle should be fine with calling
348 // get() repeatedly rather than saving the reference, and then not
349 // call get() during process shutdown.
350 static T* get(SingletonVault* vault = nullptr /* for testing */) {
351 return get_ptr({typeid(T), ""}, vault);
354 static T* get(const char* name,
355 SingletonVault* vault = nullptr /* for testing */) {
356 return get_ptr({typeid(T), name}, vault);
359 // If, however, you do need to hold a reference to the specific
360 // singleton, you can try to do so with a weak_ptr. Avoid this when
361 // possible but the inability to lock the weak pointer can be a
362 // signal that the vault has been destroyed.
363 static std::weak_ptr<T> get_weak(
364 SingletonVault* vault = nullptr /* for testing */) {
365 return get_weak("", vault);
368 static std::weak_ptr<T> get_weak(
369 const char* name, SingletonVault* vault = nullptr /* for testing */) {
370 return std::weak_ptr<T>(get_shared({typeid(T), name}, vault));
373 std::weak_ptr<T> get_weak(const char* name) {
374 return std::weak_ptr<T>(get_shared({typeid(T), name}, vault_));
377 // Allow the Singleton<t> instance to also retrieve the underlying
378 // singleton, if desired.
379 T* ptr() { return get_ptr(type_descriptor_, vault_); }
380 T& operator*() { return *ptr(); }
381 T* operator->() { return ptr(); }
383 explicit Singleton(Singleton::CreateFunc c = nullptr,
384 Singleton::TeardownFunc t = nullptr,
385 SingletonVault* vault = nullptr /* for testing */)
386 : Singleton({typeid(T), ""}, c, t, vault) {}
388 explicit Singleton(const char* name,
389 Singleton::CreateFunc c = nullptr,
390 Singleton::TeardownFunc t = nullptr,
391 SingletonVault* vault = nullptr /* for testing */)
392 : Singleton({typeid(T), name}, c, t, vault) {}
395 explicit Singleton(detail::TypeDescriptor type,
396 Singleton::CreateFunc c = nullptr,
397 Singleton::TeardownFunc t = nullptr,
398 SingletonVault* vault = nullptr /* for testing */)
399 : type_descriptor_(type) {
401 c = []() { return new T; };
403 SingletonVault::TeardownFunc teardown;
405 teardown = [](void* v) { delete static_cast<T*>(v); };
407 teardown = [t](void* v) { t(static_cast<T*>(v)); };
410 if (vault == nullptr) {
411 vault = SingletonVault::singleton();
414 vault->registerSingleton(type, c, teardown);
417 static T* get_ptr(detail::TypeDescriptor type_descriptor = {typeid(T), ""},
418 SingletonVault* vault = nullptr /* for testing */) {
419 return static_cast<T*>(
420 (vault ?: SingletonVault::singleton())->get_ptr(type_descriptor));
423 // Don't use this function, it's private for a reason! Using it
424 // would defeat the *entire purpose* of the vault in that we lose
425 // the ability to guarantee that, after a destroyInstances is
426 // called, all instances are, in fact, destroyed. You should use
427 // weak_ptr if you need to hold a reference to the singleton and
428 // guarantee briefly that it exists.
430 // Yes, you can just get the weak pointer and lock it, but hopefully
431 // if you have taken the time to read this far, you see why that
433 static std::shared_ptr<T> get_shared(
434 detail::TypeDescriptor type_descriptor = {typeid(T), ""},
435 SingletonVault* vault = nullptr /* for testing */) {
436 return std::static_pointer_cast<T>(
437 (vault ?: SingletonVault::singleton())->get_shared(type_descriptor));
440 detail::TypeDescriptor type_descriptor_;
441 SingletonVault* vault_;