1. mutable的中文翻譯是：易變的，性情不定的，跟constant（既C++中的const）是反義詞．
2. 在C++中，mutable也是為了突破const的限制而設定的。被mutable修飾的變數，將永遠處於可變的狀態，即使在一個const函式中．
   例子解釋

#include <iostream>
using namespace std;

class HuangwenMutable
{
public:
    huangwenMutable(){temp=0;}
    int Output() const
    {
        return temp++; //error C2166: l-value specifies const object
 

    }
private:
    int temp;
};

int main()
{
   HuangwenMutable huangwenMutable;
   cout<<huangwenMutable.Output()<<endl;
   return 0;
}

顯然temp++不能用在const修飾的函式裡．

#include <iostream>
using namespace std;

class HuangwenMutable
{
public:
    huangwenMutable(){temp=0;}
    int Output() const
 

    {
        return temp++; //error C2166: l-value specifies const object
    }
private:
    mutable　int temp;
};

int main()
{
   HuangwenMutable huangwenMutable;
   cout<<huangwenMutable.Output()<<endl;
   return 0;
}

計數器temp被mutable修飾，那麼它就可以突破const的限制，在被const修飾的函式裡面也能被修改．