(C++) null

 

null denotes a pointer pointing nowhere.

 

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int main() {   int * p = 0; }

 

 

 

 

 

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.

 

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#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.

 

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#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   } }

 

 

 

 

 

Advice

 

• Prefer nullptr to NULL and 0 [1]

 

 

 

 

 

References

 

1. Scott Meyers. C++ And Beyond 2012 session: 'Initial thoughts on Effective C++11'. 2012. 'Prefer nullptr to NULL and 0'