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 class and the child classes will be allowed to call the constructor.

Since only one instance of the object must...