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Tip of the Day
Language: C++
Expertise: Intermediate
Aug 6, 1999

Preventing Program's Crash due to a Pure Virtual Function's Call

A pure function should never be invoked. However, a buggy program or a buggy compiler can sometimes call a pure virtual function. In general, when a pure virtual function is invoked, the program crashes and you cannot continue debugging it. To prevent the program from crashing, you can implement the pure virtual function. This makes debugging possible even when a pure virtual function is inadvertently called.

 
#include <iostream>
using namespace std;
class Dog{
public:
  virtual void Bark()=0; //pure virtual, should never be invoked
};
  // the following definition ensures that the pure virtual call doesn't crash
void Dog::Bark(){
  cout<<"Dog::Bark called"; 
}
class Beagle : public Dog{
public:
  virtual void Bark() {cout<<"Beagle"; }
};

int main(){
  Beagle* m_pSnoopy = new Beagle;
/* The following should call Beagle::Bark(); a buggy compiler/program calls Dog::bark() instead */
  m_pSnoopy->Bark();
}

When you have fixed the bug that calls the pure virtual function (or upgrades the faulty compiler), remember to delete the implementation of the pure virtual function.

Danny Kalev
 
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