单例模式是做为"全局变量"的替代品出现的。所以它具有全局变量的特点：全局可见、贯穿应用程序的整个生命期，保证在程序运行中，某个类只存在一个实例，所以通常不希望类中的构造函数被调用

python:

class Single(object):	instance = None		#	def __init__(self):		pass	#	def getInstance(self):		if Single.instance == None:			Single.instance = Single()		return Single.instance	#	def printSelf(self):		if Single.instance != None:			print Single.instances = Single()s1 = s.getInstance()s2 = s.getInstance()s1.printSelf()s2.printSelf()</span>

C++:

Single.h

ifndef __SINGLE_H#define __SINGLE_Hclass Single{private:	static Single *instance;	Single();public:	static Single* getInstance();	void printSelf();};#endif

Single.cpp

#include "Single.h"#include <iostream>Single* Single::instance = NULL;Single::Single(){}Single* Single::getInstance(){	if (!instance)		instance = new Single;	return instance;}void Single::printSelf(){	if (instance)		std::cout << instance << std::endl;}

#include "Single.h"int main(){	Single::getInstance()->printSelf();	Single *sin = Single::getInstance();	sin->printSelf();	return  0;}

python 设计模式之 单例模式