c++ - 为什么我不能声明对可变对象的引用？ ("reference cannot be declared mutable")

Question

假设我们有一个 test.cpp 如下:

class A;

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

编译:

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

我的 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.

为什么？

Answer 1

没有理由让引用成员可变。为什么？因为 const 成员函数可以改变类成员引用的对象:

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;
};