C++核心准则T.​81：不要混用继承层级和数组

T.81: Do not mix hierarchies and arrays

T.81：不要混用继承层级和数组

Reason（原因）

An array of derived classes can implicitly "decay" to a pointer to a base class with potential disastrous results.

派生类的数组可以隐式退化为可能带来灾难性后果的指向基类的指针。

Example（示例）

Assume that Apple and Pear are two kinds of Fruits.

假定Apple和Pear是两种Fruit。

void maul(Fruit* p)
{
    *p = Pear{};     // put a Pear into *p
    p[1] = Pear{};   // put a Pear into p[1]
}

Apple aa [] = { an_apple, another_apple };   // aa contains Apples (obviously!)

maul(aa);
Apple& a0 = &aa[0];   // a Pear?
Apple& a1 = &aa[1];   // a Pear?

Probably, aa[0] will be a Pear (without the use of a cast!). If sizeof(Apple) != sizeof(Pear) the access to aa[1] will not be aligned to the proper start of an object in the array. We have a type violation and possibly (probably) a memory corruption. Never write such code.

很有可能aa[0]是一个Pear（不需要类型转化）。如果sizeof(Apple)!=sizeof(Pear)，对于aa[1]的访问位置就不会正确开始于下个对象。我们会遇到类型违反和可能的（几乎一定）内存破坏。永远不要这样写代码。

Note that maul() violates the a T* points to an individual object rule.

Alternative: Use a proper (templatized) container:

注意maul()已经违反了使用T*或onwer<T*>指明唯一对象原则。其他选项:使用适当的（模板化的）容器。

void maul2(Fruit* p)
{
    *p = Pear{};   // put a Pear into *p
}

vector<Apple> va = { an_apple, another_apple };   // va contains Apples (obviously!)

maul2(va);       // error: cannot convert a vector<Apple> to a Fruit*
maul2(&va[0]);   // you asked for it

Apple& a0 = &va[0];   // a Pear?

Note that the assignment in maul2() violated the no-slicing rule.

注意maul2()中的赋值操作违反了不要分割对象原则。

Enforcement（实施建议）

• Detect this horror! 检出这种可怕的问题！

原文链接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#t81-do-not-mix-hierarchies-and-arrays