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请写出以下程序的运行结果,并解释导致这样运行结果的关键性原因。
#include <iostream>
using std::cout;
class P
{
public:
virtual void print()
{
cout << "P";
}
};
class Q: public P
{
public:
virtual void print()
{
cout << "Q";
}
};
int main()
{
P * p = new P;
Q * q = static_cast <Q *> (p);
q->print();
delete p;
cout << endl;
q = new Q;
p = q;
q->print();
p->print();
cout << endl;
p = new (q) P;
q->print();
p->print();
cout << endl;
p->~P();
delete q;
return 0;
}