singleton

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
// T must be: no-throw default constructible and no-throw destructible
template <typename T>
struct Singleton
{
private:
struct object_creator
{
// This constructor does nothing more than ensure that instance()
// is called before main() begins, thus creating the static
// T object before multithreading race issues can come up.
object_creator() { Singleton<T>::GetInst(); }
inline void do_nothing() const {}
};
static object_creator create_object;

protected:
~Singleton() = default;
Singleton() = default;

public:
Singleton(const Singleton&) = delete;
Singleton& operator=(const Singleton&) = delete;

public:
// If, at any point (in user code), Singleton<T>::instance()
// is called, then the following function is instantiated.
static T& GetInst()
{
// This is the object that we return a reference to.
// It is guaranteed to be created before main() begins because of
// the next line.
static T obj;

// The following line does nothing else than force the instantiation
// of Singleton<T>::create_object, whose constructor is
// called before main() begins.
create_object.do_nothing();

return obj;
}
};
template <typename T>
typename Singleton<T>::object_creator Singleton<T>::create_object;
  • 删除拷贝构造函数和赋值运算符, 隐藏构造函数, 约束实例唯一.
  • 单例模板添加do_nothing函数调用, 保证单例实例构造调用在main调用之前.