先看看介绍
// 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
// }
1. 线程安全，构造成功后向AtExitManager注册，析构函数会在程序退出时调用
2. 类似Singleton，但是可以有多个同一个类型的LazyInstance，Singleton只能有一个类型的实例
3. POD https://akrzemi1.wordpress.com/2012/04/23/using-pods-in-c11/
引用了一个头文件
#include "base/dynamic_annotations.h"
下一个头文件
#include "base/at_exit.h"