c++ - 当目标指针不是基类的类型时，为什么允许dynamic_cast为多态类生成空指针？

Question

考虑以下程序

#include <iostream>
#include <iomanip>

struct A
{
};

struct C
{
};

int main()
{
    C *pc = nullptr;

    A *pa1 = dynamic_cast<A *>( pc );

    std::cout << "pa1 == nullptr is " << std::boolalpha << ( pa1 == nullptr ) << '\n';

    A *pa2 = pc;

    std::cout << "pa2 == nullptr is " << std::boolalpha << ( pa2 == nullptr ) << '\n';
}

对于两个指针声明pa1和pa2，编译器都会报告一个错误，指出不允许进行此类初始化。

例如，铛HEAD 10.0.0编译器发出以下错误。

prog.cc:19:14: error: 'C' is not polymorphic
    A *pa1 = dynamic_cast<A *>( pc );
             ^                  ~~
prog.cc:23:8: error: cannot initialize a variable of type 'A *' with an lvalue of type 'C *'
    A *pa2 = pc;
       ^     ~~
2 errors generated.

现在，让C类成为多态类。

#include <iostream>
#include <iomanip>

struct A
{
};

struct C
{
    virtual ~C() = default; 
};


int main()
{
    C *pc = nullptr;

    A *pa1 = dynamic_cast<A *>( pc );

    std::cout << "pa1 == nullptr is " << std::boolalpha << ( pa1 == nullptr ) << '\n';

    A *pa2 = pc;

    std::cout << "pa2 == nullptr is " << std::boolalpha << ( pa2 == nullptr ) << '\n';
}

而且只有第二个声明会产生错误。 dynamic_cast有效。

rog.cc:22:8: error: cannot initialize a variable of type 'A *' with an lvalue of type 'C *'
    A *pa2 = pc;
       ^     ~~
1 error generated.

允许对 dynamic_cast进行指针转换的原因是什么？

Answer 1

What is the reason of that such a conversion of pointers for the dynamic_cast is allowed?

因为 dynamic_cast在运行时运行，到那时发布编译器错误为时已晚。

如果转换失败， dynamic_cast返回 nullptr。您需要检查一下，然后在需要时进行处理。