NULL is a #defined constant for a pointer pointing nowhere. Prefer using a zero or nullptr [1].


int main()

{

  int * p = NULL; //Not preferred

  int * q =    0; //Preferred

}

Smart pointers and null

Boost smart pointers check for null themselves, so there is no need to check these to be inititialized. In the example below a member variable of a class is requested from an unitialized smart pointer. The program will abort and the runtime error will be shown.


#include <boost/scoped_ptr.hpp>

#include <boost/shared_ptr.hpp>



struct Test

{

  Test(const int x) : m_x(x) {}

  const int m_x;

};



int main()

{

  {

    boost::shared_ptr<Test> p;

    p->m_x; //Good: uninitialized pointer detected by Boost

  }

  {

    boost::scoped_ptr<Test> p;

    p->m_x; //Good: uninitialized pointer detected by Boost

  }

}

The code below shows that initializing a boost::shared_ptr with null will not be easy, but even when it succeeds, boost::shared_ptr will check itself for null. A boost::scoped_ptr can be null, but will check itself for it as well.


#include <boost/scoped_ptr.hpp>

#include <boost/shared_ptr.hpp>



struct Test

{

  Test(const int x) : m_x(x) {}

  const int m_x;

};



Test * CreateNullPointer() { return 0; }



int main()

{

  {

    boost::shared_ptr<Test> p;

    //p.reset(0); //Good: does not compile: 0 is an integer

    //p.reset(NULL); //Good: does not compile

    p.reset(CreateNullPointer()); //Bad: tricked the compiler

    //p->m_x; //Good: uninitialized pointer detected by Boost

  }

  {

    boost::scoped_ptr<Test> p;

    p.reset(0); //Valid: boost::scoped_ptr can be empty

    p.reset(CreateNullPointer()); //Valid: boost::scoped_ptr can be empty

    p->m_x; //Good: uninitialized pointer detected by Boost

  }

}

References

Joint Strike Fighter Air Vehicle C++ Coding Standards for the System Development and Demonstration Program. Document Number 2RDU00001 Rev C. December 2005. AV Rule 175: 'A pointer shall not be compared to NULL or be assigned NULL; use plain 0 instead.'

Go back to Richel Bilderbeek's C++ page.

Go back to Richel Bilderbeek's homepage.

(C++) NULL

Smart pointers and null

References