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// Copyright (c) 2008 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// The LazyInstance<Type, Traits> class manages a single instance of Type,
// which will be lazily created on the first time it's accessed. This class is
// useful for places you would normally use a function-level static, but you
// need to have guaranteed thread-safety. The Type constructor will only ever
// be called once, even if two threads are racing to create the object. Get()
// and Pointer() will always return the same, completely initialized instance.
// When the instance is constructed it is registered with AtExitManager. The
// destructor will be called on program exit.
//
// LazyInstance is completely thread safe, assuming that you create it safely.
// The class was designed to be POD initialized, so it shouldn't require a
// static constructor. It really only makes sense to declare a LazyInstance as
// a global variable using the base::LinkerInitialized constructor.
//
// LazyInstance is similar to Singleton, except it does not have the singleton
// property. You can have multiple LazyInstance's of the same type, and each
// will manage a unique instance. It also preallocates the space for Type, as
// to avoid allocating the Type instance on the heap. This may help with the
// performance of creating the instance, and reducing heap fragmentation. This
// requires that Type be a complete type so we can determine the size.
//
// Example usage:
// static LazyInstance<MyClass> my_instance(base::LINKER_INITIALIZED);
// void SomeMethod() {
// my_instance.Get().SomeMethod(); // MyClass::SomeMethod()
//
// MyClass* ptr = my_instance.Pointer();
// ptr->DoDoDo(); // MyClass::DoDoDo
// }
#ifndef BASE_LAZY_INSTANCE_H_
#define BASE_LAZY_INSTANCE_H_
#include "base/atomicops.h"
#include "base/basictypes.h"
#include "base/third_party/dynamic_annotations/dynamic_annotations.h"
namespace base {
template <typename Type>
struct DefaultLazyInstanceTraits {
static Type* New(void* instance) {
// Use placement new to initialize our instance in our preallocated space.
// The parenthesis is very important here to force POD type initialization.
return new (instance) Type();
}
static void Delete(void* instance) {
// Explicitly call the destructor.
reinterpret_cast<Type*>(instance)->~Type();
}
};
// We pull out some of the functionality into a non-templated base, so that we
// can implement the more complicated pieces out of line in the .cc file.
class LazyInstanceHelper {
protected:
enum {
STATE_EMPTY = 0,
STATE_CREATING = 1,
STATE_CREATED = 2
};
explicit LazyInstanceHelper(LinkerInitialized x) { /* state_ is 0 */ }
// Declaring a destructor (even if it's empty) will cause MSVC to register a
// static initializer to register the empty destructor with atexit().
// Check if instance needs to be created. If so return true otherwise
// if another thread has beat us, wait for instance to be created and
// return false.
bool NeedsInstance();
// After creating an instance, call this to register the dtor to be called
// at program exit and to update the state to STATE_CREATED.
void CompleteInstance(void* instance, void (*dtor)(void*));
base::subtle::Atomic32 state_;
private:
// Resets state of |helper| to STATE_EMPTY so that it can be reused.
// Not thread safe.
static void ResetState(void* helper);
DISALLOW_COPY_AND_ASSIGN(LazyInstanceHelper);
};
template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
class LazyInstance : public LazyInstanceHelper {
public:
explicit LazyInstance(LinkerInitialized x) : LazyInstanceHelper(x) { }
// Declaring a destructor (even if it's empty) will cause MSVC to register a
// static initializer to register the empty destructor with atexit().
Type& Get() {
return *Pointer();
}
Type* Pointer() {
// We will hopefully have fast access when the instance is already created.
if ((base::subtle::NoBarrier_Load(&state_) != STATE_CREATED) &&
NeedsInstance()) {
// Create the instance in the space provided by |buf_|.
instance_ = Traits::New(buf_);
CompleteInstance(instance_, Traits::Delete);
}
// This annotation helps race detectors recognize correct lock-less
// synchronization between different threads calling Pointer().
// We suggest dynamic race detection tool that "Traits::New" above
// and CompleteInstance(...) happens before "return instance_" below.
// See the corresponding HAPPENS_BEFORE in CompleteInstance(...).
ANNOTATE_HAPPENS_AFTER(&state_);
return instance_;
}
private:
int8 buf_[sizeof(Type)]; // Preallocate the space for the Type instance.
Type *instance_;
DISALLOW_COPY_AND_ASSIGN(LazyInstance);
};
} // namespace base
#endif // BASE_LAZY_INSTANCE_H_
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