[boost] singleton template template parameters

From: Jason Hise (chaos_at_[hidden])
Date: 2005-02-14 17:23:43

In my singleton code, I have been using a template template parameter in 
my lifetimes for the creator[1], so that client code does not need to 
duplicate writing the type when specifying the creation policy.  The 
drawback to this is that the creators which provide additional policies 
need to use a nested struct to avoid breaking the template signature 
that creators are required to have.  A perfect example is the 
bool_creator[2]:

template < bool Value >
struct bool_creator{
    template < typename Type >
    class creator
    {
        // pass Value to Type's ctor in create method
    };
};

If I wanted to be consistent, I would be tempted to do the same with my 
lifetime policies, separating the Type template parameter from the rest 
and having the singleton use a template template parameter for the 
lifetime.  However, I suspect this would just make client code really 
ugly when it tried to specify the lifetime it wanted to use.  In 
addition, I am willing to bet that a lot of compilers choke on template 
template parameters.

As an alternative, I am thinking of switching the creator policy to a 
non template template policy.  Although this would make client code have 
to plug the name of their class into the base singleton template yet a 
third time, I am thinking that in the end it would be more portable and 
cleaner.  Thoughts?

-Jason

[1] Yes, I made the name change from allocator to creator in my code.  
The change has not yet been uploaded though, so keep in mind that 
creators in this message refer to allocators in the current online code, 
and bool_creator is really bool_alloc at the moment.

[2] I'm thinking of dropping bool_creator from the library.  Instead of 
providing it directly, I'll develop it in the documentation as an 
example custom creator and go on to show how it can be used to implement 
a zombie singleton.  Of course, if you think it should stay in the 
library let me know, I can be persuaded to keep it.