Is this possible?
class A {
static A *instance = NULL;
public:
A () {
if (instance) {
this = instance;
} else {
instance = this;
}
}
}
Does it have leak memory?
Do I need to oveload new operator?
No. Overlooking your compiler errors, your class won't work.
@Galik has provided invaluable sources for how you'd actually want to construct a singleton. But let's look at yours.
class A {
static A *instance = NULL; // NULL isn't even a thing, but assuming you mean nullptr you can't assign a static like this
public:
A () {
if (instance) {
this = instance; // this can't be assigned
} else {
instance = this; // this is correct
}
}
};
Which would give you the following:
class A {
static A *instance;
public:
A () {
// if there's no instance, this is the one we'll use
if (!instance) {
instance = this;
}
}
};
A* A::instance = nullptr;
Which doesn't stop you from constructing more than one anyway.
Not possible. If the constructor is exposed and called, a new object of A is inevitably created. The most elegant and widely used implementations of C++ singleton classes use static methods to return the single static instance, while hiding (e.g. make private accessible) the constructor.
Here's an example:
class A {
private:
static A *instance_; // use nullptr since C++11
A() {}
public:
static A& instance() {
if (!instance_)
instance_ = new A();
return *instance_;
}
};
A* A::instance_ = nullptr;
