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;
};