Re: [boost] Cast free NULLs in algorithms

From: Michael Marcin (mmarcin_at_[hidden])
Date: 2007-07-31 17:25:59

Michael Marcin wrote:
> Chris Fairles wrote:
>> 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)
>>
> 
> Cute. It might be nice to have this as a Boost utility library.
> 

Interestingly that implementation fails for my test case because:

[temp.arg.type] 14.3.1 Template type arguments

2 A local type, a type with no linkage, an unnamed type or a type 
compounded from any of these types shall
not be used as a template-argument for a template type-parameter.

So it must be named to be useful as an argument to say std::fill.

- Michael Marcin