Re: RAII not always applicable?

restor <>
Thu, 4 Jun 2009 06:55:14 CST

I still think that you can refactor any class that has "construct dummy,
initialize later" behavior into two classes: a shared_ptr-like handler
and a _real_ class with stronger encapsulation and invariants.
Maybe the problem is sometimes developers rush to get a (relatively
less important) functionality done, and end up with one bulk class
instead of a cleaner design (I know I have :-) I would like to hear
comments on this.

If we take a plain (and perfect) example of RAII usage. Il looks like

   RAIIResource resource;
   resource.use(); /// very simple usage
} // auto-release

Now, it may be insufficient if we need to know the address of the
object before it is initialized (e.g. to pass it by reference), as in
your example:

you could use a smart pointer, or a custom wrapper:
std::shared_ptr<T> val;
std::cin >> val;

First, I would use boost::optional rather than std::shared_ptr, unless
shared ownership was really required. The former doesn't allocate
anything on the heap and is therefore faster.
But the problem that still remains is that whereas T can now be a full
RAII objct, the usage is still a two-phase init with all its burden:

   std::shared_ptr<T> val; // dummy init
   std::cin >> val; // perhaps full init

   if( val ) { // defensive if required :(

You could call this pattern "Making two-phase init out of single-phase
init class". It is very useful in some situations. I found it useful
once when implementing a problem solver. You would request searching
for the best solution (but usually it means infinately long) and
request a stop and return of the best solution found so far:

   void OnButtonSolve() {

   void OnButtonStop() {
     Solution sol = solver.stop();
     store( sol );

Functions 'start' and 'stop' were respectively creating and joining
+destroying a boost::thread, which is somewhat RAII, in-place with the
help of boost::optional.

About OP's second example... A real-world example would help in finding
a better design.

The example I provided is from the being-developed Boost library:
Boost.Coroutine. You can obtain it from Boost Vault:

I didn't want to quote too much in the post. It has a good
documentation, and the example is taken literarly from there.


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