The mutable storage class specifier (C++ only)

The mutable storage class specifier is used only on a class data member to make it modifiable even though the member is part of an object declared as const. You cannot use the mutable specifier with names declared as static or const, or reference members.

In the following example:

class A
{
  public:
    A() : x(4), y(5) { };
    mutable int x;
    int y;
};

int main()
{
  const A var2;
  var2.x = 345;
  // var2.y = 2345;
}

the compiler would not allow the assignment var2.y = 2345 because var2 has been declared as const. The compiler will allow the assignment var2.x = 345 because A::x has been declared as mutable.