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

References can only be assigned when constructing an object, and cannot be modified thereafter. Thus making them mutable would have no meaning, which is why the standard disallows it.