Singleton

来源：Lintcode

原帖：http://www.lintcode.com/en/problem/singleton/

题目：
Singleton is a most widely used design pattern. If a class has and only has one instance at every moment, we call this design as singleton. For example, for class Mouse (not a animal mouse), we should design it in singleton.
You job is to implement a getInstance method for given class, return the same instance of this class every time you call this method.

Have you met this question in a real interview? Yes
Example
In Java:
A a = A.getInstance();
A b = A.getInstance();
a should equal to b.

Challenge
If we call getInstance concurrently, can you make sure your code could run correctly?

代码：

 class Solution {  
 public:  
   /**  
    * @return: The same instance of this class every time  
    */  
   static Solution* getInstance() {  
     // write your code here  
     if (obj == NULL) {  
       obj = new Solution();  
     }  
     return obj;  
   }  
 private:  
   Solution() {}  
   static Solution* obj;  
 };  
 Solution* Solution::obj = NULL;