[boost] Need opinion on implementation issue

From: Daryle Walker (darylew_at_[hidden])
Date: 2008-08-19 18:20:20

I have a class that has the cool pubic member access without "()",  
like "left" and "right" for boost::bimap.  In my case, it's meant to  
simulate a function object, so it can be passed to functions like  
std::for_each, and the return value from said functions (the updated  
function object) can be assigned back to the special member.  This  
function object forwards to the actual computation object within my  
class object.

I implemented this by making a special inner private class and have  
the auxiliary function object be a public data member of that private  
type.  The actual computation object isn't directly in my class  
object, but within the public auxiliary member.  So the auxiliary  
function object can be passed by copy or assignment and everything  
works.

The problem is in a class derived from my first one.  It also has a  
public auxiliary function object data member.  However, it forwards  
to the SAME actual computation object as the base class's auxiliary  
member.  I implemented this by the derived class's auxiliary keeping  
a pointer to the computation object within the base's auxiliary.  The  
issue is when I create temporary derived-auxiliaries for use in  
std::for_each.  I have to create a copy of the computation object  
with new and delete on destruction.  The derived-auxiliary keeps a  
bool flag indicating whether the pointer points to a contained object  
or a heap one.  I was thinking of making a boost::variant<CompObj,  
CompObj*> instead, but trades use of the heap for temporaries for  
making all derived-auxiliaries huge (if sizeof(CompObj) > sizeof 
(CompObj*)):

Current: base's CompObj + derived's CompObj* and bool
New: base's CompObj + derived's variant
   -> base's CompObj + derived's max(CompObj, CompObj*)
   -> 2 * CompObj
(None of these include the various administrative padding or  
"otherwise empty" bytes.)

I haven't changed it yet because I suspect this is a bad trade-off.   
But what happens on systems with low/no heaps?  Should this be some  
sort of macro switch?

-- 
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT hotmail DOT com