Re: [boost] singleton raw pointer access

From: Jason Hise (chaos_at_[hidden])
Date: 2005-04-28 13:08:15

Rob Stewart wrote:

>As I understand it, the operation is inherently unsafe; it isn't
>a matter of efficiency, or even desirable usage.  Given that, it
>is most appropriate that you spell things differently.  You want
>to call attention to such usage.
>  
>
Even though the usage is undesirable, it may still be necessary in cases 
where the singleton needs to behave polymorphically like one of its base 
classes.

>Having said that, is it really the case that operator *()
>couldn't do the same work as operator ->() (possibly adding an
>exception if you can't get the underlying object)?
>  
>
The difference is that the pointer returned by operator -> is guaranteed 
to be used immediately, and by using an intermediate pointer class I can 
construct a lock for the duration of the member call (this technique is 
described in Modern C++ Design, in the smart pointer section).

Operator * is incapable of constructing this type of temporary lock, and 
additionally there is no guarantee that the reference returned would 
used and discarded immediately.  Once the actual reference is returned, 
no more checking can take place.

>Assuming you still don't provide operator *(), or you also want
>the raw semantics of your get_unsafe_*() function(s), I wonder if
>cast notation wouldn't be even better (versus providing access as
>a public member function):
>
>   T * q(unsafe<T *>(p));
>   T & r(unsafe<T &>(p));
>
>That is, unsafe() (whatever you might call it) would always take
>a singleton pointer, but depending upon whether it was
>parameterized to get a pointer or reference would do something
>different.
>
I do like that syntax.  Thanks for the suggestion, I'll see if I can 
make that work.

-Jason