Forget to explicitly define a virtual destructor in polymorphic base classes.

[ltimaginea]

CPP/week12/examples/virtual.cpp

Lines 5 to 14 in 14c7055

	class Person
	{
	public:
	string name;
	Person(string n): name(n){}
	virtual void print()
	{
	cout << "Name: " << name << endl;
	}
	};

The class Person doesn't have a user-written virtual destructor, so its destructor defaults to public non-virtual.

CPP/week12/examples/virtual.cpp

Lines 49 to 51 in 14c7055

	Person * p = new Student("xue", "2020");
	p->print(); //if print() is not a virtual function, different output
	delete p; //if its destructor is not virtual

The result of delete p; is undefined behavior, so it might call Person::~Person only and leak the string id .

In general, we should explicitly define a virtual destructor in polymorphic base classes. See C.127: A class with a virtual function should have a virtual or protected destructor .