C++中dynamic_pointer_cast的作用是什么？ 什么时候使用？ dynamic_pointer_cast 的替代方案是什么？

Question

I was reading about dynamic_pointer_cast. I found that we can dynamically upcast or downcast the shared pointer of one type to another at run time but i did't understand the concept completely. Shall both the classes must have same properties? If the properties must be same then what is need of dynamic casting?

Answer 1

dynamic_pointer_cast is used to convert std::shared_ptr type, eg from the pointer on a base class to a pointer on a derived class:

#include <memory>

struct A{ virtual ~A() = default; };
struct B: A {};

int main()
{
    std::shared_ptr<A> pA = std::make_shared<B>();
    std::shared_ptr<B> pB = std::dynamic_pointer_cast<B>(pA);
}

So dynamic_pointer_cast is a convenience wrapper around dynamic_cast :

template< class T, class U > 
std::shared_ptr<T> dynamic_pointer_cast( const std::shared_ptr<U>& r ) noexcept
{
    if (auto p = dynamic_cast<typename std::shared_ptr<T>::element_type*>(r.get())) {
        return std::shared_ptr<T>{r, p};
    } else {
        return std::shared_ptr<T>{};
    }
}