Mutable keyword

mutable member-variable-declaration;

This keyword can only be applied to non-static and non-const data members of a class. If a data member is declared mutable, then it is legal to assign a value to this data member from a const member function.

上面的引自MSDN
举个例子:
#include <iostream>
#include <iomanip>

using namespace std;

class C
{

public:
 C(int i):m_Count(i){}
 int incr() const//注意这里的const
 {
  return ++m_Count;
 }
 int decr() const
 {
  return --m_Count;
 }
private:
 mutable int m_Count;//你可以试着将这里的mutable去掉再编译试试
};

void main()
{
 C c1(0),c2(10);
 for(int tmp,i=0;i<10;i++)
 {
  tmp = c1.incr();
  cout<<setw(tmp)<<setfill(' ')<<tmp<<endl;
  tmp = c2.decr();
  cout<<setw(tmp)<<setfill(' ')<<tmp<<endl;
 }
}