In several of my books the mutable example uses a non-const member function incorrectly as was pointed out by Paul Ezust (see for example C++ for C Programmers:3rd edition p156.) The indicated change corrects the problem. The following code is a small variation on the book that incorporates the correction. 

#include <  iostream  >
#include <  string  >
using  namespace std;

class person {
public:
  person(const string n, int a, const unsigned long ss)
      :name(n), age(a), soc_sec(ss){}
  void bday() const  {++age;}  //had been non-const
  void print() const
  { 
     cout << name << " is  " << age 
     << "  years old with SSN  " << soc_sec << endl;
  }
private:
  const string name;
  mutable int age;
  const unsigned long soc_sec;
};

int main()
{
   const person ira("ira pohl", 38, 1110111UL);
   ira.print();
   ira.bday();
   ira.print();

}