Can any one provide me a sample of Singleton in c++?
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() {
}
您可以避免使用像这样的静态对象来删除它：
if(m_pA == 0) {
static A static_instance;
m_pA = &static_instance;
}
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:
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
C ++中的单例可以这样写：
static A* A::GetInstance() {
static A sin;
return &sin;
}