Singletonis 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 agetInstancemethod 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?

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

Solution* Solution::instance = NULL;