When to use virtual destructor.

main.cpp

#include <iostream>
#include <sstream>
#include <memory>

class A {
 public:
  A() {
    std::ostringstream oss;
    oss << "Constructor A@" << std::hex << (void *)this << "\n";
    std::cout << oss.str() << std::flush;
  }

  ~A() {
    std::ostringstream oss;
    oss << "Destructor A@" << std::hex << (void *)this << "\n";
    std::cout << oss.str() << std::flush;
  }
};

class B : public A {
 public:
  B() {
    std::ostringstream oss;
    oss << "Constructor B@" << std::hex << (void *)this << "\n";
    std::cout << oss.str() << std::flush;
  }

  ~B() {
    std::ostringstream oss;
    oss << "Destructor B@" << std::hex << (void *)this << "\n";
    std::cout << oss.str() << std::flush;
  }
};

class VA {
 public:
  VA() {
    std::ostringstream oss;
    oss << "Constructor VA@" << std::hex << (void *)this << "\n";
    std::cout << oss.str() << std::flush;
  }

  virtual ~VA() {
    std::ostringstream oss;
    oss << "Destructor VA@" << std::hex << (void *)this << "\n";
    std::cout << oss.str() << std::flush;
  }
};

class VB : public VA {
 public:
  VB() {
    std::ostringstream oss;
    oss << "Constructor VB@" << std::hex << (void *)this << "\n";
    std::cout << oss.str() << std::flush;
  }

  virtual ~VB() {
    std::ostringstream oss;
    oss << "Destructor VB@" << std::hex << (void *)this << "\n";
    std::cout << oss.str() << std::flush;
  }
};

int main(int argc, char const *argv[]) {
  std::cout << "======================= Stack Test =======================" << std::endl;
  {
    A a;
    B b;
  }
  {
    VA va;
    VB vb;
  }
  std::cout << "======================= Heap Test =======================" << std::endl;
  {
    A *b = new B();
    delete b; // IMPORTANT: Resource Leak Here!
  }
  {
    VA *vb = new VB();
    delete vb;
  }
  std::cout << "======================= SmartPtr Test =======================" << std::endl;
  {
    std::shared_ptr<A> pB = std::make_shared<B>();
  }
  {
    std::shared_ptr<VA> pVB = std::make_shared<VB>();
  }
  std::cout << "======================= End Test =======================" << std::endl;
  return 0;
}