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

This could blow your mind away, but a reference is never mutable (can't be made to refer to another object) and the referenced value is always mutable (unless you have a reference-to-const):

#include 

struct A
{
  int& i;
  A(int& n): i(n) {}
  void inc() const 
  {
    ++i;
  }
};

int main()
{
  int n = 0;
  const A a(n);
  a.inc();
  std::cout << n << '\n';
}

A const method means that a top-level const-qualifier gets added to the members. For a reference this does nothing (= int & const a;), for a pointer it makes the pointer, not the pointee const (= int* const p, not const int* p;).