动态转换不为非多态基类工作？ [英] dynamic cast not working for non polymorphic base class?

问题描述

这里，第二个转换错误说明

Here the second cast gives an error saying

cast.cc:35:35：error：can not dynamic_cast'base' 'class CBase *'）to type'class CDerived *'（source type is not polymorphic）

    CBase * base = new CDerived; 
    CBase* pb;
    CDerived * der = new CDerived; 
    CDerived* pd;
    pb = dynamic_cast<CBase*>(der);     // ok: derived-to-base
    pd = dynamic_cast<CDerived*>(base);  // wrong: base-to-derived

这个是什么意思

为什么这么做如果我使基类多态？

And why this works if I make the base class polymorphic ?

有人可以让我知道这背后的基本概念。

Can some one please let me know the basic concept behind this.

推荐答案

因为标准说的是这样（见C ++标准中的[expr.dynamic.cast]部分）：

Because the standard says so (see the section [expr.dynamic.cast] in the C++ standard):

... dynamic_cast< T>（v）

... v 应为多态类型的指针或值。

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

在实践中，由于运行时类型信息（RTTI）需要进行动态展开（即从基本到派生）可能与 vtbl / vptr机制一起生成，如果没有多态成员函数，则不需要。

In practice, because the run-time type information (RTTI) required to make dynamic down-casts (i.e. from base to derived) possible are generated along with the vtbl/vptr mechanism, which isn't required if there are no polymorphic member functions.

另一方面，向上扩展（即派生为base） RTTI（没有运行时决定做）。引用标准的同一部分：

Up-casts (i.e. derived to base), on the other hand, require no RTTI (there's no run-time decision to be made). Quoting from the same section of the standard:

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

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