Re: [boost] Cast free NULLs in algorithms

From: Chris Fairles (chris.fairles_at_[hidden])
Date: 2007-07-31 14:22:51

This was a proposed library implementation of nullptr for c++0x from
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1601.pdf

const                        // this is a const object...
class {
public:
  template<class T>          // convertible to any type
    operator T*() const      // of null non-member
    { return 0; }            // pointer...
  template<class C, class T> // or any type of null
    operator T C::*() const  // member pointer...
    { return 0; }
private:
  void operator&() const;    // whose address can't be taken
} nullptr = {};              // and whose name is nullptr

Very similar to your impl although I think the static_cast in yours is
redundant. The standard says, 0, being an integral constant expression
that evaluates to zero, is a null pointer constant which is implicitly
convertible to a pointer type (4.10)

Chris

On 7/31/07, Michael Marcin <mike_at_[hidden]> wrote:
> Hello,
>
> I have something like:
>
> class A;
> boost::array<A*,10> the_array;
>
> std::fill( the_array.begin(), the_array.end(), NULL );
>
> Which wont compile (at least under Visual Studio 2005) without a cast
> for NULL... I've had this problem before and decided to try to find an
> easy cast free expressive solution for it.
>
> After a few minutes I came up with:
>
> namespace detail
> {
>      struct null_ptr_impl
>      {
>          template< typename T >
>          operator T* () const { return static_cast<T*>(0); }
>      };
> } // end namespace detail
> const detail::null_ptr_impl null_ptr;
>
> std::fill( the_array.begin(), the_array.end(), null_ptr );
>
> Does this exist somewhere already? Is it a good solution?
>
> Thanks,
>
> Michael Marcin
>
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