const_cast示例

const_cast用于对"const"或"volatile"进行转换。

代码1

int main()
{
    const int i = 0;
    int *j;
    j = const_cast<int*>(&i);

    cout << &i << " "<< j << endl;
    cout << i << " " << *j << endl;

    *j = 1;

    cout << &i << " "<< j << endl;
    cout << i << " " << *j << endl;
    return 0;
}

结果1

0xbf9dec84 0xbf9dec84
0 0
0xbf9dec84 0xbf9dec84
0 1

本来想通过const_cast得到的指针，修改i的内容，但结果并没有成功。i的值还是0，跟同学讨论，估计是编译对const值进行了优化，直接在程序中替换了。

代码2

int main()
{
    const int i = 0;
    const int &ii = const_cast<int &>(i);
    int *j;
    j = const_cast<int*>(&ii);

    cout << &ii << " "<< j << endl;
    cout << ii << " " << *j << endl;
    

    *j = 1;

    cout << &ii << " "<< j << endl;
    cout << ii << " " << *j << endl;
    cout << i << endl;
    return 0;
}

结果2

0xbf5d9084 0xbf5d9084
0 0
0xbf5d9084 0xbf5d9084
1 1
0

i的值还是没有改变。

代码3

class A {
    public:
        int val;
    public:
        A(int v) : val(v) {
        };
};


int
main() {
    const A a(1);
    A* p = const_cast<A *>(&a);

    p->val = 2;
    cout << &a << " " << p << endl;
    cout << a.val << endl;
    cout << p->val << endl;
}

结果3

0xbf5fd8a8 0xbf5fd8a8
2
2

对象a的val值被改变了，是因为类对象没有被优化么？？