Re: [boost] singleton: zombie_call

From: Jason Kankiewicz (jkankiewicz_at_[hidden])
Date: 2005-07-26 13:50:19

Jason Hise wrote:
> I am trying to come up with a mechanism which allows member functions of 
> the singleton to be called if the instance exists, and which do nothing 
> if the instance does not exist (no creation is attempted, the operation 
> is simply skipped).  My current idea is a member function of the 
> singleton pointer named zombie_call, which would be used something like:
> 
> ptr.zombie_call ( &MySingletonType::SomeFunc, param1, param2, ... );
> 
> The only problem I have is what to do if the instance does not exist and 
> the member function has a return value.  Should I just return a 
> default-constructed object in these cases?  (And what should happen if 
> the return type does not have a constructor taking no arguments?)  
> Should I append a parameter to the end of zombie_call which is returned 
> if the instance does not exist, and have it default to a 
> default-constructed object?  Should I append a parameter that is a 
> reference to the return type, and just assign the return value to it if 
> the operation is successful?  (And what happens if the return type is 
> already a reference?)  Should I just enforce that member functions 
> called with zombie_call return void?
> 
> Also, is there any way to make the syntax of the call a bit nicer?  I 
> was originally hoping to have a separate pointer type called zombie_ptr, 
> which just behaved in this manner when functions were accessed via 
> operator->, but I couldn't think of a non-intrusive way to pull it off...
> 
> Any and all thoughts are appreciated.
Regarding how a call to the singleton class' static "Instance()" method 
should be handled when no instance of the singleton exists, couldn't the 
class take a "lifetime" policy class template parameter which determines 
whether a new instance is constructed or an exception should be thrown?