Complaints from common member functions and mutable

In C ++ member functions, a member function is called a common member function, for example, int get () const;

It is recorded in the encyclopedia that a common member function can only read the values of data members in the same class, but cannot be modified.

So what if it is modified? Of course we get a compilation error ....

What if I have to modify it? Then, mutable appears.

The mutable keyword is used to modify the value of a constant function.

Examples of mutable:

 

class Employee {public:    Employee(const std::string & name)         : _name(name), _access_count(0) { }    void set_name(const std::string & name) {        _name = name;    }    std::string get_name() const {        _access_count++;        return _name;    }    int get_access_count() const { return _access_count; }private:    std::string _name;    mutable int _access_count;}

 

class MathObject {public:    MathObject() : pi_cached(false) { }    double pi() const {        if( ! pi_cached ) {            /* This is an insanely slow way to calculate pi. */            pi = 4;            for(long step = 3; step < 1000000000; step += 4) {                pi += ((-4.0/(double)step) + (4.0/((double)step+2)));            }            pi_cached = true;        }        return pi;    }private:    mutable bool pi_cached;    mutable double pi;};