Re: Which design in better?

Nitesh <>
Tue, 26 Aug 2008 00:45:36 -0700 (PDT)
On Aug 26, 11:02 am, Paavo Helde <> wrote:

Nitesh <> kirjutas:

Objects of type X are basic building blocks of my application and
there are a few functions which return XHandle (typedef X** XHandle).
The task at hand needs to use a couple of these functions and then
iterate over X objects.
Because its done frequently and it separates iteration from task-at-
hand'logic we decided to create iterator.

Assuming the availability of functions GetXHandleSize(XHandle) and
DisposeXHandle(XHandle) there are two alternatives.

<b>Option1:</b> Using simple inheritance
class XHandleIterator {
 XHandleIterator(void) : mHandle(0L), mCount(0) {}
 ~XHandleIterator() { if (mHandle != 0L) DisposeXHandle(mHandle); }

 bool Done(void) { return mCount > 0; }
 // Pre-increment operator
 XHandleIterator& operator++(void) { --mCount; return *this; } // Am
iterating over backwards
 // Post-increment operator..
 // De-referencing operator
 X& operator *(void) { return (*mHandle)[mCount-1]; }
 XHandle mHandle;
 int mCount;

class XHandleIteratorOne : public XHandleIterator {
 XHandleIteratorOne(SomeType obj) {
 // Creates XHandle someway. Assigns to mHandle and mCount

class XHandleIteratorTwo : public XHandleIterator {
 XHandleIteratorTwo(SomeType obj) {
 // Creates XHandle in another way. Assigns to mHandle and mCount

<b>Option2:</b> Policy based design
template<typename HandleCreator>
class XHandleIterator {
 XHandleIterator (SomeType obj) : mHandle(0L), mCount(0)
  HandleCreator::Create(obj, &mHandle);
  mCount = GetXHandleSize(mHandle)/sizeof(X);
 ~XHandleIterator() { if (mHandle != 0L) DisposeXHandle(mHandle); }
 bool Done(void) { return mCount > 0; }
  // Pre-increment operator
 XHandleIterator& operator++(void) { --mCount; return *this; }
 // Post-increment operator...
 // De-referencing operator
 X& operator *(void) { return (*mHandle)[mCount-1]; }
 XHandle mHandle;
 int mCount;

struct XHandleCreator1 {
     static void Create(SomeType obj, XHandle *handle) {
         // Creates XHandle someway. Assigns to handle

struct XHandleCreator2 {
     static void Create(SomeType obj, XHandle *handle) {
         // Creates XHandle in another way. Assigns to handle

typedef XHandleIterator<XHandleCreator1> XHandleIteratorOne;
typedef XHandleIterator<XHandleCreator2> XHandleIteratorTwo;

I am confused over which is the right way to do in this case with all
stipulations stated at the top. The main thing I like about 2nd one is
that mHandle and mCount are private.
Please comment on which of the above alternative is better?


It appears you want the iterator to perform two tasks: iterate over the
array, and dispose the array when done. It would be cleaner to keep these
tasks separate IMO. To follow the STL pattern there should be a
"container" class, from where the iterators can be obtained by begin()
and end() member functions. The fact that the data is actually living
elsewhere does not have any significance. The "container" object will
call Dispose in its destructor.

The usage would go like this:

XContainer my_xvector = ObtainXHandleInWhatEverWay();

for(XContainer::iterator p = my_xvector.begin();
        p != my_xvector.end(); ++p) {

        X& xref = *p;
        // Do whatever with X


I see no need for inheritance or policy-based templates here, based on
the info you provided. The container object should just have a
constructor and/or an assignment operator taking XHandle as a parameter,
regardless of in which way the handle was obtained. Mixing up this class
with different ways to obtain the handle would just convolute the design.
If really needed, one could provide a separate "factory" class for taking
care of obtaining the handle in different ways.


This is thing I need to do
1) Obtain handle
2) Iterate over the handle
3) Dispose the obtained handle

For my task its the 1st (obtaining handle) that differs. 2nd and 3rd
(iterating over and disposing) are always done in same way so I put
them in same class.

But with your explanation it seems reasonable not to mix 2nd and 3rd.
So, obtaining and disposing handle should be responsibility of one
class and iterating over that should be done by another but related

How about having one XContainer base class that has a nested public
Iterator class that knows how to iterate over the XContainer? And
there will be derived classes OR template specializations
XContainerOne/Two that know how to obtain the handle. Again which of
these two is better?


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