c++ - 从基类转换到派生类抛出异常

Question

我有 3 个继承类:Base -> Intermediate -> Derived。

为什么我使用dynamic_cast会抛出异常？

class Base { ... };
class Intermediate : public Base { ... };
class Derived : public Intermediate { ... };

Base* base = new Derived();

// No throw
auto intermediate = static_cast<Intermediate *>(base);
auto derived1 = static_cast<Derived *>(base);
auto derived2 = static_cast<Derived *>(intermediate);

// All throw
// (vcruntime140d.dll): Access violation reading location [...].
auto intermediate = dynamic_cast<Intermediate *>(base);
auto derived1 = dynamic_cast<Derived *>(base);
auto derived2 = dynamic_cast<Derived *>(intermediate);

Answer 1

当转换为指向派生类的指针时，dynamic_cast 的操作数应该是指向多态类型 的指针，即声明或继承虚拟类型的类函数，而 static_cast 没有这个约束。

作为返回，如果操作数实际上并未指向目标类型对象的子对象，则 dynamic_cast 会抛出异常，而 static_cast 则未定义。