Why can't I declare a reference to a mutable object? (“reference cannot be declared mutable”)

Question

Let\'s say we have a test.cpp as follows:

class A;

class B
{
    private:
        A mutable& _a;
};

Compilation:

Answer 1

There is no reason to have a reference member mutable. Why? Because const member functions can change the object which is referenced by a class member:

class B {
public:
    B(int var) : n(var) {};
    void Set(int val) const { n = val; }  //no error
    void SetMember(int val) const { m = val; }  //error assignment of member `B::m' in read-only structure
protected:
    int& n;
    int m;
};