使用unique_ptr的多态类的C++static_cast和dynamic_cast

我在C++中的多态类上练习static_cast和dynamic_cast。我尝试了使用原始指针和unique_ptr。前者不会制造问题，后者会制造问题。在这里我展示我的代码：-

#include <iostream>
#include <memory>
#include <exception>
#include <stdexcept>
using namespace std;
class A
{
    int a, id=0;
    static int i;
public:
    A()
    {
        id=++i;
        cout<<"constructing A: "<<id<<"n";
    }
    virtual void get()
    {
        cout<<"enter a: ";
        cin>>a;
    }
    virtual void disp()
    {
        cout<<"a = "<<a<<"n";
    }
    virtual ~A()
    {
        cout<<"destroying A: "<<id<<"n";
    }
};
int A::i=0;
class B: public A
{
    int b;
public:
    B()
    {
        cout<<"constructing Bn";
    }
    void get()
    {
        cout<<"enter b: ";
        cin>>b;
    }
    void disp()
    {
        cout<<"b = "<<b<<"n";
    }
    ~B()
    {
        cout<<"destroying Bn";
    }
};
void show (unique_ptr<B> &p)
{
    p->get();
    p->disp();
}
void d_cast (unique_ptr<A> &pa)
{
    unique_ptr<B> pb;
    try
    {
        pb.reset(dynamic_cast<B*>(pa.release()));
        if (pb==nullptr)
            throw runtime_error {"nullptr exception"};
        show(pb);
        cout<<"dynamic_cast successfulnn";
    }
    catch (exception &e)
    {
        cout<<"dynamic_cast unsuccessful: "<<e.what()<<"nn";
    }
    pa.reset(pb.release());
}
void s_cast (unique_ptr<A> &pa)
{
    unique_ptr<B> pb;
    try
    {
        pb.reset(static_cast<B*>(pa.release()));
        if (pb==nullptr)
            throw runtime_error {"nullptr exception"};
        show(pb);
        cout<<"static_cast successfulnn";
    }
    catch (exception &e)
    {
        cout<<"static_cast unsuccessful: "<<e.what()<<"nn";
    }
    pa.reset(pb.release());
}
int main()
{
    cout<<R"(using "unique_ptr<A> pa with new A" :-)"<<"nn";
    unique_ptr<A> pa(new A);    // (1)
    d_cast(pa);
    s_cast(pa);      // (2)
    cout<<"n"<<R"(using "unique_ptr<A> pa with new B" :-)"<<"nn";
    pa.reset(new B);
    d_cast(pa);
    s_cast(pa);
    return 0;
}

代码的输出为：-

using "unique_ptr<A> pa with new A" :-
constructing A: 1
dynamic_cast unsuccessful: nullptr exception
static_cast unsuccessful: nullptr exception

using "unique_ptr<A> pa with new B" :-
constructing A: 2
constructing B
enter b: 7
b = 7
dynamic_cast successful
enter b: 8
b = 8
static_cast successful
destroying B
destroying A: 2

我只有两个问题，因为我已经标记：-

1. 为什么第一个对象{用（1）表示}没有被破坏，而用"新B"调用的对象却被破坏了？

2. 为什么（2）抛出异常？有趣的是，如果我反转s_cast(pa)和d_cast(pa)的位置，那么（2）不会抛出任何异常，并且工作正常（然而，问题（1）仍然存在）。