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

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; }