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.

mutable member-variable-declaration;

For example, the following code will compile without error because m_accessCount has been declared to be mutable, and therefore can be modified by GetFlag even though GetFlag is a const member function.

// mutable.cpp
class X
{
public:
   bool GetFlag() const
   {
      m_accessCount++;
      return m_flag;
   }
private:
   bool m_flag;
   mutable int m_accessCount;
};

int main()
{
}

Keywords