Can any one provide me a sample of Singleton in c++?

Question

I write a singleton c++ in the follow way:

class A {     private:         static A* m_pA;         A();         virtual ~A();      public:         static A* GetInstance();         static void FreeInstance();          void WORK1();         void WORK2();         void WORK3();     } }  A* A::GetInstance() {     if (m_pA == NULL)         m_pA = new A();     return m_pA; }  A::~A() {     FreeInstance()  // Can I write this? are there any potential error? }  void A::FreeInstance() {     delete m_pA;     m_pA = NULL; } 

Thanks! Evan Teran and sep61.myopenid.com 's answer is right, and really good! My way is wrong, I wish any one writting such code can avoid my silly mistake.

My singleton A in my project has a vector of smart pointer, and another thread can also edit this vector, so when the application is closing, it always become unstable even I add lots of CMutex. Multithread error + singleton error wasted me 1 day.

//----------------------------------------------------------- A new singleton, you are welcome to edit if you think there is any problem in the following sample:

class A {     private:         static A* m_pA;         explicit A();         void A(const A& a);         void A(A &a);         const A& operator=(const A& a);         virtual ~A();      public:         static A* GetInstance();         static void FreeInstance();          void WORK1();         void WORK2();         void WORK3();     } }  A* A::GetInstance() {     if (m_pA == NULL){         static A self;         m_pA = &self;     }     return m_pA; }  A::~A() { } 

Answer 1

Why does everybody want to return a singleton as a pointer?
Return it as a reference seems much more logical!

You should never be able to free a singleton manually. How do you know who is keeping a reference to the singleton? If you don't know (or can't guarantee) nobody has a reference (in your case via a pointer) then you have no business freeing the object.

Use the static in a function method.
This guarantees that it is created and destroyed only once. It also gives you lazy initialization for free.

class S {     public:         static S& getInstance()         {             static S    instance;             return instance;         }     private:         S() {}         S(S const&);              // Don't Implement.         void operator=(S const&); // Don't implement  }; 

Note you also need to make the constructor private. Also make sure that you override the default copy constructor and assignment operator so that you can not make a copy of the singleton (otherwise it would not be a singleton).

Also read:

• https://stackoverflow.com/a/1008289/14065
• Singleton: How should it be used
• C++ Singleton design pattern

To make sure you are using a singleton for the correct reasons.

Though technically not thread safe in the general case see:
What is the lifetime of a static variable in a C++ function?

GCC has an explicit patch to compensate for this:
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg00265.html

Answer 2

You can avoid needing to delete it by using a static object like this:

if(m_pA == 0) {     static A static_instance;     m_pA = &static_instance; }