求高人指教，C++ const_cast问题
OS：redHat5
compile：g++
先贴代码
#include <iostream>
using namespace std;
int main()
{
const int a = 97;
int &b = const_cast<int&>(a);
b = 64;
cout<<"b = "<<b<<endl;
cout<<"a = "<<a<<endl;
const string str("ttttttt");
string &cstr = const_cast<string &>(str);
cstr = "fffffff";
cout<<"str = "<<str<<endl;
cout<<"cstr = "<<cstr<<endl;
return 0;
}
编译没有错误，运行结果：
b = 64
a = 97
str = fffffff
cstr = fffffff
问题：为什么string类型的const对象经过const_cast转换后，可以对const对象的值进行改变，而int型对象的值却没有发生变化
