RE: [boost] boost::nil

From: Chris Little (cslittle_at_[hidden])
Date: 2001-06-08 08:21:13

Doug,

Unfortunately adding nil will prevent anyone who is building Macintosh
software from using Boost.

On a Mac, nil is a preprocessor macro included in the system headers as a
legacy of the days when the OS had a Pascal interface.

Chris

> -----Original Message-----
> From: Douglas Gregor [mailto:gregod_at_[hidden]]
> Sent: Thursday, June 07, 2001 9:06 PM
> To: boost_at_[hidden]
> Subject: [boost] boost::nil
> 
> 
> Hello all,
> 	The function library currently under review contains a 
> type 'nil_t' with an 
> instance 'nil'. It is a replacement for "0" or "NULL" in any 
> code. The 
> implementation is trivial, but it should probably be placed 
> in its own header 
> because it's usefulness extends beyond 'function'. The code is just:
> 
> namespace boost {
>   struct nil_t 
>   {
>     template<typename T> operator T*() const { return 0; }
>     template<typename T, typename U> operator T U::*() const 
> { return 0; }
>   };
> 
>   template<typename T>
>   inline bool operator==(const T* p, const nil_t&) { return !p; }
> 
>   template<typename T>
>   inline bool operator==(const nil_t&, const T* p) { return !p; }
> 
>   template<typename T>
>   inline bool operator!=(const T* p, const nil_t&) { return p; }
> 
>   template<typename T>
>   inline bool operator!=(const nil_t&, const T* p) { return p; }
> 
>   namespace {
>     nil_t nil;
>   }
> }
> 
> Advantages of 'nil' over NULL:
> 	- NULL is not necessarily portable. Some C++ compilers 
> pick up bad 
> definitions of NULL from the system C libraries, making it useless.
> 	- The type of NULL is unspecified, so it is impossible 
> to overload a 
> function based on it.
> 
> Advantages of 'nil' over 0:
> 	- 0 is an integer. It converts to pointer types but 
> doesn't convey much 
> information to the user.
> 	- the type of 0 is "const int". Overloading a function 
> to take a null value 
> based on const int also allows any integer value to be passed 
> in, and the 
> error cannot be detected at compile-time.
> 
> 	Doug