Re: [boost] Re: auto_set / state_saver

From: Jason Hise (chaos_at_[hidden])
Date: 2005-03-15 11:17:36

Sérgio Vale e Pace wrote:

>>>template < typename T >
>>>struct auto_set
>>>{
>>>    T & var; T val;
>>>    auto_set ( T & var, T val ) :
>>>      
>>>
>>auto_set(T & var,T val = T()):
>>// allows one to just specify a variable to be auto-set back to its default
>>value also
>>    
>>
>>>    var ( var ), val ( val ) {  }
>>>      
>>>
>
>// IMHO I thing it´s more often useful to use var own value to be
>restored later:
>auto_set(T& var) : var(var), val(var) { }
>
>  
>
>>>    ~ auto_set (  ) { var = val; }
>>>};
>>>      
>>>

Perhaps.  You might be interested to see why I originally wrote it though:

virtual void create (  )
{
    if ( !s_inst && !creating )
    {
        creating = true;
        auto_set < bool > reset ( creating, false );
        s_inst = creator_inst.create (  );
    }
}

virtual void destroy (  )
{
    if ( s_inst && !destroying )
    {
        destroying = true;
        auto_set < bool > reset_d ( destroying, false );
        auto_set < raw_pointer > reset_p ( s_inst, 0 );
        creator_inst.destroy ( s_inst );
    }
}

Basically, when create is called, the constructor of the singleton being 
created just might do something that would normally cause that singleton 
to be automatically created.  I use a bool to remember that a creation 
is in progress, and auto_set makes sure that it gets turned off when the 
scope is exited.

The same kind of thing is going on in destroy, but I also ensure that 
the pointer to the singleton gets reset to null after destruction is 
complete.

-Jason