mutable 是不是如下这么用？会出错

[unhappyless_14]

#include<iostream>
using namespace std;
class testmutable
{
public:
testmutable(int i=0):num(i){}
void say()
{
cout<<"const function"<<num<<endl;
}

private:
mutable int num; // 用在这
};

int main()
{
const testmutable one;
one.say();
}

[taodm]

出啥错？
另外，楼主，你在用啥书学C++？
找c＋＋ Primer，看书后索引表，查mutable

[v2002750]

一个const对象能调用一个非const的方法吗？

[v2002750]

#include<iostream>
using namespace std;
class testmutable
{
public:
testmutable(int i=0):num(i){}

void say()
{
cout<<"const function"<<num<<endl;
}

public:
mutable int num; // 用在这
};

void main()
{
const testmutable one;
one.num=5;
}

[lann64]

#include<iostream>
using namespace std;
class testmutable
{
public:
testmutable(int i=0):num(i){}
void say() const
{
num++; //一样可以修改num的值。
cout<<"const function: "<<num<<endl;
}

private:
mutable int num; // 用在这
};

int main()
{
const testmutable one;
one.say();
}
这样用。
你的say函数忘了用const，对于const对象当然编译通不过。

[missilery]

Mutable Data Members
It sometimes (but not very often) happens that a class has a data member that we want to be able to modify, even inside a const member function. We can indicate such members by declaring them as mutable.

A mutable data member is a member that is never const, even when it is a member of a const object. Accordingly, a const member function may change a mutable member. To declare a data member as mutable, the keyword mutable must precede the declaration of the member:

class Screen {
public:
// interface member functions
private:
mutable size_t access_ctr; // may change in a const members
// other data members as before
};



We've given Screen a new data member named access_ctr that is mutable. We'll use access_ctr to track how often Screen member functions are called:

void Screen::do_display(std::ostream& os) const
{
++access_ctr; // keep count of calls to any member function
os << contents;
}



Even though do_display is const, it can increment access_ctr. That member is a mutable member, so any member function, including const functions, can change the value of access_ctr.

[OOPhaisky]

mutable的作用是：让一个数据成员可以在const成员函数中被修改。
class testmutable
{
public:
testmutable(int i=0):num(i){}
void say()const//注意这里的const，说明此成员函数是const的
{
num = 3;//由于num是const的，所以可以在此const成员函数中进行修改
}

private:
mutable int num;
};

[OOPhaisky]

还要提醒楼主，不可以用const和mutable同时修饰一个data member:
class testmutable
{
public:
//......

private:
const mutable int num; //error:const和mutable不可以同时用
};

[北极猩猩]

呵呵，来晚了。

补充一点
通常mutable的成员都是不会影响对象的状态的数据，换句话说，只有有它没它不影响类的外部功能，这个成员才是合作mutable

[missilery]

http://book.devworld.cn/book/download.asp?softid=228&downid=0&id=545
去下载一<C++ Primer中文版>电子书