C++'s mutable and conceptual constness

c++ 中的 mutable 关键字是什么意思？

mutable is for the case where an object is logically constant, but in practice needs to change. 

 

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;
};

 

More details: 

http://www.highprogrammer.com/alan/rants/mutable.html