LCOV - code coverage report
Current view: directory - ipc/chromium/src/base - lazy_instance.h (source / functions) Found Hit Coverage
Test: app.info Lines: 13 13 100.0 %
Date: 2012-06-02 Functions: 17 14 82.4 %

       1                 : // Copyright (c) 2008 The Chromium Authors. All rights reserved.
       2                 : // Use of this source code is governed by a BSD-style license that can be
       3                 : // found in the LICENSE file.
       4                 : 
       5                 : // The LazyInstance<Type, Traits> class manages a single instance of Type,
       6                 : // which will be lazily created on the first time it's accessed.  This class is
       7                 : // useful for places you would normally use a function-level static, but you
       8                 : // need to have guaranteed thread-safety.  The Type constructor will only ever
       9                 : // be called once, even if two threads are racing to create the object.  Get()
      10                 : // and Pointer() will always return the same, completely initialized instance.
      11                 : // When the instance is constructed it is registered with AtExitManager.  The
      12                 : // destructor will be called on program exit.
      13                 : //
      14                 : // LazyInstance is completely thread safe, assuming that you create it safely.
      15                 : // The class was designed to be POD initialized, so it shouldn't require a
      16                 : // static constructor.  It really only makes sense to declare a LazyInstance as
      17                 : // a global variable using the base::LinkerInitialized constructor.
      18                 : //
      19                 : // LazyInstance is similar to Singleton, except it does not have the singleton
      20                 : // property.  You can have multiple LazyInstance's of the same type, and each
      21                 : // will manage a unique instance.  It also preallocates the space for Type, as
      22                 : // to avoid allocating the Type instance on the heap.  This may help with the
      23                 : // performance of creating the instance, and reducing heap fragmentation.  This
      24                 : // requires that Type be a complete type so we can determine the size.
      25                 : //
      26                 : // Example usage:
      27                 : //   static LazyInstance<MyClass> my_instance(base::LINKER_INITIALIZED);
      28                 : //   void SomeMethod() {
      29                 : //     my_instance.Get().SomeMethod();  // MyClass::SomeMethod()
      30                 : //
      31                 : //     MyClass* ptr = my_instance.Pointer();
      32                 : //     ptr->DoDoDo();  // MyClass::DoDoDo
      33                 : //   }
      34                 : 
      35                 : #ifndef BASE_LAZY_INSTANCE_H_
      36                 : #define BASE_LAZY_INSTANCE_H_
      37                 : 
      38                 : #include "base/atomicops.h"
      39                 : #include "base/basictypes.h"
      40                 : 
      41                 : namespace base {
      42                 : 
      43                 : template <typename Type>
      44                 : struct DefaultLazyInstanceTraits {
      45            4260 :   static void New(void* instance) {
      46                 :     // Use placement new to initialize our instance in our preallocated space.
      47                 :     // The parenthesis is very important here to force POD type initialization.
      48            4260 :     new (instance) Type();
      49            4260 :   }
      50            4257 :   static void Delete(void* instance) {
      51                 :     // Explicitly call the destructor.
      52            4257 :     reinterpret_cast<Type*>(instance)->~Type();
      53            4257 :   }
      54                 : };
      55                 : 
      56                 : // We pull out some of the functionality into a non-templated base, so that we
      57                 : // can implement the more complicated pieces out of line in the .cc file.
      58                 : class LazyInstanceHelper {
      59                 :  protected:
      60                 :   enum {
      61                 :     STATE_EMPTY    = 0,
      62                 :     STATE_CREATING = 1,
      63                 :     STATE_CREATED  = 2
      64                 :   };
      65                 : 
      66            5856 :   explicit LazyInstanceHelper(LinkerInitialized x) { /* state_ is 0 */ }
      67                 :   // Declaring a destructor (even if it's empty) will cause MSVC to register a
      68                 :   // static initializer to register the empty destructor with atexit().
      69                 : 
      70                 :   // Make sure that instance is created, creating or waiting for it to be
      71                 :   // created if neccessary.  Constructs with |ctor| in the space provided by
      72                 :   // |instance| and registers dtor for destruction at program exit.
      73                 :   void EnsureInstance(void* instance, void (*ctor)(void*), void (*dtor)(void*));
      74                 : 
      75                 :   base::subtle::Atomic32 state_;
      76                 : 
      77                 :  private:
      78                 :   DISALLOW_COPY_AND_ASSIGN(LazyInstanceHelper);
      79                 : };
      80                 : 
      81                 : template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
      82                 : class LazyInstance : public LazyInstanceHelper {
      83                 :  public:
      84            5856 :   explicit LazyInstance(LinkerInitialized x) : LazyInstanceHelper(x) { }
      85                 :   // Declaring a destructor (even if it's empty) will cause MSVC to register a
      86                 :   // static initializer to register the empty destructor with atexit().
      87                 : 
      88                 :   Type& Get() {
      89                 :     return *Pointer();
      90                 :   }
      91                 : 
      92           25555 :   Type* Pointer() {
      93           25555 :     Type* instance = reinterpret_cast<Type*>(&buf_);
      94                 : 
      95                 :     // We will hopefully have fast access when the instance is already created.
      96           25555 :     if (base::subtle::NoBarrier_Load(&state_) != STATE_CREATED)
      97            4260 :       EnsureInstance(instance, Traits::New, Traits::Delete);
      98                 : 
      99           25555 :     return instance;
     100                 :   }
     101                 : 
     102                 :  private:
     103                 :   int8 buf_[sizeof(Type)];  // Preallocate the space for the Type instance.
     104                 : 
     105                 :   DISALLOW_COPY_AND_ASSIGN(LazyInstance);
     106                 : };
     107                 : 
     108                 : }  // namespace base
     109                 : 
     110                 : #endif  // BASE_LAZY_INSTANCE_H_

Generated by: LCOV version 1.7