Example showing why C++ base class destructor should be virtual

virtual_base_dtor.cpp

// Example to show why base class destructor should be virtual.
//
// If ~A is not virtual below, you will notice that B's destructor
// is not called and B's Foo object is not freed. Memory leak!
//
// Change ~A to virtual and Foo is freed at end of scope correctly.

#include <iostream>
#include <memory>
using namespace std;

class Foo
{
    public:
    Foo()  { cout << "Foo ctor\n"; }
    ~Foo() { cout << "Foo dtor\n"; }
};

// Base class
class A
{
    public:
    A()  { cout << "A ctor\n"; }
    // Note: Make this virtual to prevent memory leak
    ~A() { cout << "A dtor\n"; }
};

// Derived class
class B : public A
{
    public:
    B()
    {
        cout << "B ctor\n";
        fp = unique_ptr<Foo>(new Foo);
    }
    ~B() { cout << "B dtor\n"; }
    unique_ptr<Foo> fp;
};

int main()
{
    {
        // Take a base class pointer to derived class object
        unique_ptr<A> a(new B);
    }
    cout << "Scope ended\n";

    return 0;
}

// If ~A is not virtual, output is:
// A ctor
// B ctor
// Foo ctor
// A dtor
// Scope ended

// If ~A is virtual, output is:
// A ctor
// B ctor
// Foo ctor
// B dtor   <-- B destructor called
// Foo dtor <-- Foo object destroyed
// A dtor
// Scope ended