為什么父類的析構函數定義成virtual后， delete pTest2 就會釋放Derived的資源， 否則就只能釋放Base的資源， 而delete pTest1就能釋放Derived 和Base的資源class Base{public:     ~Base() {        cout << "Base destructor!" << endl;     }; };class Derived: public Base {public:     ~Derived() {        cout << "Derived destructor!" << endl;     }; };int main() {     Derived *pTest1 = new Derived();     Base *pTest2 = new Derived();    delete pTest1;    delete pTest2;    return 0; }