假设我们有一个test.cpp如下：
class A;
class B
{
private:
A mutable& _a;
};
汇编：
$> 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?
没有理由让引用成员可变。为什么？因为const成员函数can更改类成员引用的对象：
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;
};
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