c++ - 为什么多态性不适用于 C++ 中的数组？

Question

这个问题在这里已经有了答案:

C++ new[] into base class pointer crash on array access (5 个回答)

关闭 5年前。

#include <iostream>

using namespace std;

struct Base
{
    virtual ~Base()
    {
        cout << "~Base(): " << b << endl;
    }

    int b = 1;
};

struct Derived : Base
{
    ~Derived() override
    {
        cout << "~Derived(): " << d << endl;
    }

    int d = 2;
};

int main()
{
    Base* p = new Derived[4];
    delete[] p;
}

输出如下:(Visual Studio 2015 with Clang 3.8)

~Base(): 1
~Base(): 2
~Base(): -2071674928
~Base(): 1

为什么多态性不适用于 C++ 中的数组？

Answer 1

给定，

Base* p = Derived[4];

C++11 标准制定

delete [] p;

是未定义的行为。

5.3.5 Delete

...

2 ... In the second alternative (delete array) if the dynamic type of the object to be deleted differs from its static type, the behavior is undefined.

从内存布局的角度来看，delete [] p; 的原因也是有道理的。将导致未定义的行为。

如果 sizeof(Derived)是 N , new Derived[4]分配的内存类似于:

+--------+--------+--------+--------+
|   N    |   N    |   N    |   N    |
+--------+--------+--------+--------+

一般来说，sizeof(Base) <= sizeof(Derived) .在您的情况下，sizeof(Base) < sizeof(Derived)自从 Derived有一个额外的成员变量。

使用时:

Base* p = new Derived[4];

你有:

p
|
V
+--------+--------+--------+--------+
|   N    |   N    |   N    |   N    |
+--------+--------+--------+--------+

p+1指向自 sizeof(Base) < sizeof(Derived) 以来第一个对象中间的某个位置.

       p+1
       |
       V
+--------+--------+--------+--------+
|   N    |   N    |   N    |   N    |
+--------+--------+--------+--------+

当在 p+1 上调用析构函数时，指针不指向对象的开始。因此，该程序表现出未定义行为的症状。

---

相关问题

由于 Base 的大小不同和 Derived ，您不能使用 p 遍历动态分配数组的元素.

for ( int i = 0; i < 4; ++i )
{
   // Do something with p[i]
   // will not work since p+i does not necessary point to an object
   // boundary.
}