Transforming SysInfo into a singleton

Promises are made to be kept: we will now transform the SysInfo class into a singleton. C++ offers many ways to implement the singleton design pattern. We will explain one of them here. Open the SysInfo.h file and make the following changes:

class SysInfo 
{ 
public: 
    static SysInfo& instance(); 
    virtual ~SysInfo(); 
 
    virtual void init() = 0; 
    virtual double cpuLoadAverage() = 0; 
    virtual double memoryUsed() = 0; 
 
protected: 
    explicit SysInfo(); 
 
private: 
    SysInfo(const SysInfo& rhs); 
    SysInfo& operator=(const SysInfo& rhs); 
}; 

The singleton must guarantee that there will be only one instance of the class and that this instance will be easily accessible from a single access point.

So the first thing to do is to change the visibility of the constructor to protected. This way, only this...