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(C++) null

 

null denotes a pointer pointing nowhere.

 

int main()
{
  int * p = 0;
}

 

 

 

 

 

Boost 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
  }
}

 

 

 

 

 

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