Re: [boost] null

From: Jason Hise (chaos_at_[hidden])
Date: 2005-07-28 22:29:53

Eric Niebler wrote:

> [...redefining NULL breaks things...]
> <>struct foo { foo(void*) {} };
> void bar(foo f) {}
> void baz() {
> bar(NULL);
> }
>
> Works with NULL as 0, fails with NULL as null because it needs two
> user-defined conversions.
>
> Redefining NULL is a career-limiting move. Don't do it.

Alright, I concede that I shouldn't redefine a standard macro.  :-P  
Interesting example by the way... hadn't thought of that.  Would be nice 
if there was a way around it.  I find the upper limit on user defined 
implicit conversions frustrating.  This limit has already thwarted my 
plan to make a smart reference type which only locks the contained 
instance while being implicitly converted to the raw reference type.  It 
would have worked something like:

template < typename Type >
struct Ref
{
    struct InternalRef
    {
        Lock my_lock;
        operator Type & (  );
    };
    operator InternalRef (  );
};

This was planned to work similar to the locking pointer idiom:

template < typename Type >
struct Ptr
{
    struct InternalPtr
    {
        Lock my_lock;
        Type * operator -> (  );
    };
    InternalPtr * operator -> (  );
};

But sadly, the locking reference idea requires two implicit user defined 
conversions.  And I can't exactly overload the '.' operator.  *sigh*  
Why does the C++ language have to ban such potentially fun things as 
infinite implicit conversion chains and overloading the dot operator?

-Jason