C ++ dynamic_cast - 多态性要求和向下转换 [英] C++ dynamic_cast - polymorphic requirement and downcasting

问题描述

在下面的代码中，当 obj 在 case 1 中构建派生类，但它的成员函数是不可访问的 obj 。因此，虽然下
铸造（即在情况2），使用 obj 作为源，我们任何如何构造的 code>中。为什么需要 obj 需要是多态的？

In the following code, while construction of obj in case 1 we would any how construct derived class too but it's member functions are just inaccessible to obj. So while down- casting ( i.e., in case 2 ) , using obj as source, we any how has the constructed derived in it. Why would it require obj needs to be polymorphic?

如果我把你和上面的描述混淆， obj 不需要是多态的，但是当使用 dynamic_cast 时需要下载它。

If I confused you with my above description, Why in upcast obj need not to be polymorphic but while downcast it needs to be while using dynamic_cast ?

class base
{
    public:
    base()
    {
        cout<< " \n base constructor \n";
    }
};

class derived:public base
{
    public:
    derived()
    {
         cout <<" \n derived constructor \n";
    }
};

base *obj = dynamic_cast<base*> (new derived) ; // case 1: explicitly upcasting
derived *OBJ = dynamic_cast<derived*> (obj) ;   // case 2: error

谢谢。

推荐答案

从5.2.7 / 1 [expr.dynamic.cast]：

From 5.2.7/1 [expr.dynamic.cast] :

的表达式 dynamic_cast< T>（v）是将表达式v转换为类型
T的结果。

The result of the expression dynamic_cast<T>(v) is the result of converting the expression v to type T.

[...]

如果T是指向 cv1 B的指针， cv2 D，使得B是D的基类，结果是指向由v指向的D对象的唯一B子对象的
指针。

If T is "pointer to cv1 B" and v has type "pointer to cv2 D" such that B is a base class of D, the result is a pointer to the unique B sub-object of the D object pointed to by v.

[...]

否则，v应是指向多态类型的指针或左值。

Otherwise, v shall be a pointer to or an lvalue of a polymorphic type.

标准甚至提供了以下示例，说明多态类型要求不代表派生到基本转换：

The standard even provides the following example which illustrates that the polymorphic type requirement does not stand for derived to base conversion :

struct B {};
struct D : B {};
void foo(D* dp)
{
    B* bp = dynamic_cast<B*>(dp); // equivalent to B* bp = dp;
}

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