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:

$> gcc test.cpp test.cpp:6:20: error: reference ‘_a’ cannot be declared ‘mutable’ [-fpermissive] 

My gcc:

$> gcc --version gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1 Copyright (C) 2011 Free Software Foundation, Inc. This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 

Why?

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; };