python 设计模式之 单例模式

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，保证在程序执行中，某个类仅仅存在一个实例，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.instance

s = Single()
s1 = s.getInstance()
s2 = s.getInstance()
s1.printSelf()
s2.printSelf()</span>

C++:

Single.h

ifndef __SINGLE_H
#define __SINGLE_H

class 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;
}