Singleton 单例模式

常见设计模式之单例模式

//: C10:CuriousSingleton.cpp
// Separates a class from its Singleton-ness (almost).
#include <iostream>
using namespace std;

template<class T> class Singleton {
  Singleton(const Singleton&);
  Singleton& operator=(const Singleton&);
protected:
  Singleton() {}
  virtual ~Singleton() {}
public:
  static T& instance() {
    static T theInstance;
    return theInstance;
  }
};

// A sample class to be made into a Singleton
class MyClass : public Singleton<MyClass> {
  int x;
protected:
  friend class Singleton<MyClass>;
  MyClass() { x = 0; }
public:
  void setValue(int n) { x = n; }
  int getValue() const { return x; }
};

int main() {
  MyClass& m = MyClass::instance();
  cout << m.getValue() << endl;
  m.setValue(1);
  cout << m.getValue() << endl;

  return 0;
} ///:~

步骤：

MyClass is made a Singleton by:
1.  Making its constructor private or protected. 构造函数受保护
2.  Making Singleton<MyClass> a friend. 授权访问受保护的构造
3.  Deriving MyClass from Singleton<MyClass>. 继承